If you believe the best way to get Yosemite information is to interrogate the entrance gate attendant, extracting several minutes of detailed information while the melodious tooting of automobile horns grows behind you, then this page is not for you. Really, this entire website would qualify as a spoiler alert, so it's best to move on, perhaps to an online shop that vends Kevlar vests.
If, however, you're a planner ahead, you'll find at least some of the books & maps listed here worthwhile. If you're not in a hurry, most of the items listed below are available in various gift shops in Yosemite.
This is the map I use when I'm hiking in Yosemite. It's a topographic map (elevations listed in feet) that shows all the hiking trails in Yosemite, including distances in miles for each segment. It's large - 37 by 25 inches unfolded, with maps on both sides - and includes detail maps of Yosemite Valley and Tuolumne Meadows. It's also highly durable and waterproof; I've been using my original copy for years.
The Photographer's Guide to Yosemite · Michael Frye
This guide, available as a book or iPhone/iPad app, is an excellent resource for photographers planning a prize-winning visit to Yosemite. It describes dozens of likely locations, including directions, best times (of year and day) for shooting, and high-quality sample photographs from each suggested site. It's also sprinkled with general photography advice on topics like composition and depth of field, and includes an appendix with tables of sunrise/sunset times throughout the year. The book is sturdy and small enough to toss in your backpack or camera bag. The app version, besides containing essentially the same content as the book, can calculate the sunrise/sunset times, moonrise/moonset times, phase of the moon, and path of the sun and moon across Yosemite Valley for any date.
The author also writes a terrific blog devoted to Yosemite photography. He posts lots of firsthand information on seasonal information of interest to photographers, such as fall color and the Horsetail Fall firefall, does regular critiques of reader-submitted photos, and as you might expect, includes lots of striking photos.
The Laws Field Guide to the Sierra Nevada · John Muir Laws
This guide is a remarkable achievement, with thousands of watercolor illustrations, all hand-painted by the author. It lacks the Sierra Nevada Natural History's detailed descriptions, but its brief descriptions are all on the same pages as the illustrations they describe, saving you some page turning. Besides all the usual categories you'd expect to find in a natural history guide, like plants, mammals, birds, mushrooms, etc., it includes unexpected categories like bats, galls, animal scat & tracks, and the night sky.
Yosemite Wildflowers App · Judy and Barry Breckling
This app, available from the Apple and Google, stores, covers more than 500 species of wildflower that grow inside Yosemite, including descriptions, multiple photos of each species, and explanations of how to distinguish Yosemite's many flowers that have near identical twins (or, as is often the case, triplets, quadruplets, etc.). You can browse by common or scientific name, search for specific names, or narrow your search results based on one or more of a dozen plant characteristics such as flower color, leaf shape, and month of bloom. The photography isn't always high quality, but it's certainly good enough for identification purposes. If you're already going to be carrying a phone with you in Yosemite, this is your best option for an on-the-trail flower guide, and it's cheaper (eight dollars as of this writing) than most paper guides.
Sierra Nevada Wildflowers · Elizabeth L. Horn
This wildflower guide contains photos of around 260 species. It's organized by plant family rather than by color, so you'll be able to find what you're looking for much faster if you already have some flower knowledge. It includes an index with both common and scientific plant names, plus a brief glossary of botanical lingo.
Sierra Nevada Wildflowers · Karen Wiese
This guide, organized by flower color, includes photos of just under 300 wildflower species. As with the other book of the same name, it contains a complete index plus a glossary and a description of the basic Sierra habitats (foothills, mixed coniferous, subalpine, etc.).
Wildflowers of Yosemite · Lynn Wilson, Jim Wilson, and Jeff Nicholas
This guide, also organized by flower color, includes photos of 224 species. Unlike all the other wildflower books listed here, the photos and descriptions of each flower aren't together - the photos are all in the first half of the book, and the descriptions in the second half. The book includes common & scientific-name indices, plus a region-by-region overview of the park listing a sampling of the flowers you can expect to find in each area.
Wildflowers of the Sierra Nevada and Central Valley · Laird R. Blackwell
This guide contains photos of more than 350 species of wildflower. The flower photos/description section is organized by flower family, but there's also a thumbnail guide at the beginning of the book with smaller photos of each flower, organized by elevation and subcategorized by flower color and number of petals. This is the most thorough of any of the wildflower books except the huge Botti/Sydoriak tome (which definitely won't fit into your backpack), but the photography is generally not of high quality.
An Illustrated Flora of Yosemite National Park · Stephen J. Botti & Walter Sydoriak
It uses illustrations instead of photos, and there appears to have been a crisis in the colored-ink budget about three hundred pages into the project. Nonetheless, this lap-breaker of a book is the authority for Yosemite wildflowers, with more than 1,300 illustrations, most of them in color. Besides the usual descriptive detail, it suggests scouting grounds for each plant and is rich with quirky flower-world lingo like "lanceolate" and "involucre." I usually use the photo guides in the other books to identify a flower, then come to this book to get the goods on its inner life and scandalous past.
Wildflowers of California · Larry Ulrich & Susan Lamb
There's little descriptive detail here, you'll only find about half the species in the Yosemite environs, and this isn't a throw-it-in-the-pack field guide. It's also the only wildflower book on this list without an index. If all you're after is gorgeous flower photos, though, this is the book for you. The quality of the photographs (of which there are about 130) is superb and outstrips any of the other wildflower guides listed here.
Off the Wall: Death in Yosemite · Michael P. Ghiglieri and Charles R. Farabee, Jr.
This macabre but fascinating volume chronicles every unnatural death in Yosemite from the park's appearance in recorded history until the book went to press in 2007. It includes Indian battles; unfortunate cases like Giacomo Campi, who in 1871 stepped back to bow to a passing lady and fell 35 feet to his death; and hundreds of other demises. Reading this book will certainly convince you not to try wading into the waters of the Mist Trail. You'll also learn, among other things, that auto accidents are the leading killer in the park, and that no one in Yosemite has ever been killed by a mammalian predator (bears, mountain lions, coyotes, etc.).
Sierra Nevada Natural History · Tracy I. Storer, Robert L. Usinger, and David Lukas
A useful reference if you want to know what kind of critter is staring back at you from the Yosemite woods. It's a grab bag of color photos and descriptions, with categories including fungi (17), lichens and mosses (10), ferns and miscellaneous plants (20), flowering plants (82), shrubs (35), trees (42), miscellaneous animals and spiders (9), insects (47), moths and butterflies (57), fishes (23), amphibians (13), reptiles (14), birds (108), and mammals (47). It's about the size of the Audubon Society Field Guides. The main drawback is that the color plates don't mention the page numbers where the descriptions can be found, leaving you to thumb through the book to find them (although the reverse isn't true - the descriptions do reference color plate numbers).
Day Hikes In Yosemite National Park: 80 Great Hikes · Robert Stone
This book contains descriptions and simple, useful maps for 80 day hikes in Yosemite, but includes no photographs. It's light enough to throw in your backpack for a hike and isn't printed on expensive glossy paper that you'd feel bad about getting dog-eared. Weight-wise, this book and Suzanne Swedo's are the best carry-along options.
100 Hikes in Yosemite National Park · Marc J. Soares
In addition to covering the most hikes (although a couple dozen of them aren't actually in Yosemite), this guide has easy-to-understand 3/4 overhead perspective maps and is the only hiking guide with color photography. The hikes described here are a mix of day hikes and backpacking trips.
Hiking Yosemite National Park · Suzanne Swedo
This guide reviews 65 hikes, a mixture of day and multi-day trails. Besides hike descriptions, it includes simple maps, elevation tables, and black & white photography. It's light enough to carry in your backpack.
Top Trails Yosemite · Jeffrey P. Schaffer
Mr. Schaffer publishes Yosemite hiking books at a dizzying rate, but at least for the moment, this is his latest effort. It covers 40+ day hike & backpacking trips in Yosemite, including maps, elevation tables, and black & white photography. It's heavier than the Robert Stone & Suzanne Swedo books, and you may not want to carry it around on the trail with you. Schaffer also publishes a more complete book, Yosemite National Park: A Complete Hikers Guide, that covers 83 hikes.
One Best Hike: Yosemite's Half Dome · Rick Deutsch
It's unusual to read an entire book to prepare for a single hike, but Half Dome is an unusual hike. The author has hiked Half Dome many times and doesn't stint with his knowledge, covering topics like what gear to use, training regimens, a point-by-point summary of the hike, and even a description of the landmarks you'll pass if you drive to Yosemite from the bay area. The author also blogs about Yosemite-related news.