Yosemite is home to over 1300 species of flowering plants and over 1400 "taxa". Only a handful of these are non-native and most of those are found only below 6,000 feet. Furthermore, because of the elevation range in the park and its near environs, flowers are in bloom somewhere from March through September.
When it comes to books, the variation here is greater than in any other category. One trail guide is a lot like another, but this category runs the gamut from weighty tomes intended for botanists (the Jepson Manual; also available online), to even weightier tomes intended for flower enthusiasts (Botti's Illustrated Flora of Yosemite National Park), to small field guides. All have their pitfalls and frustrations. The ones for botanists are frustrating for us laymen (wait, are those superior or inferior ovaries?). If you don't know your calyx from your sepal, those books aren't for you. The small books, on the other hand, have few species and sometimes not even much overlap from one book to the next. The identifications are ultimately based on imprecise factors like the color and number of petals.
For starters, if you have already purchased the Laws Field Guide (ahem, like I told you!), you have a pretty good flower book already. If you're a first time visitor to Yosemite, that will give you a nice overview of about 500 of the most common species. Laws gives fantastic practical tips for telling similar species apart in the field. This is far and away the easiest flower book to use, plus you get all the other stuff (ferns, reptiles, insects, scat, etc). Quite simply, buy this book first. It's that simple. Everyone I know loves it and you won't really need another book for a while.
For a second book, you might be tempted to get something like the Audubon Society Field Guide to North American Wildflowers: Western Region. Don't. At least not if your goal is to identify plants in Yosemite. It was my first flower book and, fine book though it may be, it's just that it covers such a vast area, that it ultimately only has a tiny percentage of the flowers you''ll find in the central Sierra.
Stephen Botti, Illustrated Flora of Yosemite National Park
This is a massive, coffee table book with descriptions and illustrations of every known species of flowering plant in Yosemite, including not trees, shrubs, grasses and sedges. That's over 1300 species! Botti gives a dichotomous key and a complete treatment to each flower, but he tries to use field-recognizable characteristics when possible. You'll need considerable technical expertise to use the key, but unlike a botanist's key, it doesn't require you to bring the seeds into the lab and put them under a microscope in order to get an identification. So this is for the flower enthusiast who would like to think himself a botanist. For me, it is often difficult to use still, but as I get better at recognizing families and so forth, it becomes more useful as an identifier. Where it really shines is when you think you have a rough idea at what family or even genus a plant belongs to, you can find out if there are similar species that might lead you to make a mistaken identification. Also, since he lists common locations, known altitudes and more, you can often rule out flowers based on those type of criteria. Finally, and perhaps most significantly, this is a beautiful book, sumptuously illustrated with over a thousand original illustrations. If you like plants, you can thumb through this all winter long. I imagine it will be out of print soon, but available on the used market for some time and currently Amazon has it used for 40% of what I paid for it. I have to say, I'm surprised to find used copies of this book. Most people I know who have gone to the trouble to buy it consider it a treasured possession, but I suppose people get it and realize that it weighs a ton (can't bring it with you) and is not easy to use for neophytes (no classifications by color and number of petals here).
Jim and Lynn Wilson, Wildflowers of Yosemite
The Complete Yosemite Flora is amazing, of course, but it's no field guide. As with the trail guides, every book has its strengths, but I especially like Wildflowers of Yosemite by Jim and Lynn Wilson. It's small, cheap, and Yosemite-focussed. One of my first evaluations for a flower book for Yosemite is whether it includes Sierra Lessingia (Lessingia leptoclada), because most books don't include it and yet it is one of the dominant roadside species, a flower you absolutely can't miss if you're in Yosemite (or King's Canyon for that matter) in late summer. I find it frustrating to pass miles and miles of a flower on the roadsides and not find it in my field guide. This book has it (and so does the Laws Field Guide). I suspect that many books take a pass on this one because it's a non-native, but I want the major, dominant, obvious species in my field guides, native or not. I want to know what I'm seeing first, and then learn more about it.
The Wilson book also has a unique approach in that it puts together a series of wildflower itineraries, most of which include some driving and some walking. They give suggestions on specifically when and where to see specific species and you can use it as a roadside guide as you come into the park and drive along the roads. If one of your goals in visiting Yosemite is to see wildflowers, you should have this book and the Laws Field Guide up in the front seat and accessible so that you can use them as guides while entering the park. Having the Wilson's itineraries keyed to your odometer mileage from known points (entrance gates or major junctions) not only helps you see those species, but also identify them when you do. It's a fun guide that prompts you to get out and explore. There are a couple of other worthwhile flower books, to be sure, but start with this one and Laws, then buy Botti. If that doesn't quench your thirst, drop me an email.