These are my favorite books on plants and their uses, and they're mainly intended for my area, eastern North America. There are many more books out there for whatever region you live in. I've linked to Powell's Books, which is a fantastic independent bookstore in Portland, Oregon, but you can find these books through your local independent bookstore with Booksense if you prefer.

Identification

Newcomb's Guide to Wildflowers, Lawrence Newcomb
An excellent guide to small plants, arranged by flower shape and number of petals, with an extremely helpful key in the contents. Includes a line drawing for each plant and color plates for several species. Includes practically every species found in the area, and has excellent sections on goldenrods and other confusing groups.

Peterson Field Guide to Wildflowers: Northeastern and North-Central North America, Roger Tory Peterson
A good, comprehensive guide to small plants, arranged by color, with brief but helpful descriptions and a line drawing of each plant. Includes almost every species of small plant you're likely to encounter in this area, and also includes shrubs and small trees with showy flowers.

Peterson Field Guide to Eastern Trees, George A. Petrides
A nice tree guide with color photos, line drawings, and range maps, covering most species found in Eastern and Central North America .

National Audubon Society Field Guide to Wildflowers: Eastern Region, William A. Niering
I don't like the confusing way the book is split between color plates and plant listings, but it's good to have another guide to fall back on, with slightly different descriptions.

National Audubon Society Field Guide To North American Trees: Eastern Region, Elbert E. Little, Jr.
Same as above, but for trees.

Uses

Peterson's Guide to Edible Plants, Lee Allen Peterson
An excellent guide to the food uses of plants, including pretty much every edible plant species in eastern North America. It has excellent line drawings for identification, and color plates for some species. The explanations aren't extensive (see Bradford Angier's guide below for fuller listings of edible uses), but this is an excellent book for identification and basic information on the uses of many plants.

Peterson's Guide to Medicinal Plants of Eastern/Central North America, Steven Foster
Includes many plants, with extensive accounts of their medicinal uses. Unfortunately, the latest edition of this book replaced the line drawings with photos, which I find rather unhelpful for identification: there's usually only one photo per plant, and it's usually a close up of the flower, which is useless if you're trying to identify the plant by its leaves. I suggest getting the older edition if you can find it, or carrying along a Newcomb's and a Peterson Tree Guide for good line drawings.

Tom Brown's Field Guide to Wild Edible and Medicinal Plants, Tom Brown, Jr.
An intimate guide to 44 important edible and medicinal plants. The stories Brown tells about his experiences with these plants are fascinating; rather than just listing uses like most plant books, he explains how he first "met" the plant and how he learned of its uses from his teacher, Stalking Wolf. You really get a sense of the plant's "personality". His Field Guide to Wilderness Survival also includes a section on utilitarian uses of plants.

Stalking the Wild Asparagus, Euell Gibbons
An excellent book with detailed entries on many plant groups, including personal stories of Gibbons' experiences, and instructions on making foods, such as jams and preserves. Another great book is Stalking the Healthful Herbs.

Field Guide to Edible Wild Plants, Bradford Angier
More than 100 plants, with extensive descriptions for identification, detailed descriptions of their uses, and beautiful color illustrations. Also includes folk name index. Includes the most common and useful food plants, like oak, dandelion, and nettles, but not the more obscure ones. For a more complete listing, check the Peterson guide.

Field Guide to Medicinal Wild Plants, Bradford Angier
The same as above, but for medicinal plants.

Cunningham's Encyclopedia of Magical Herbs, Scott Cunningham
An excellent book on the magical uses of many plants, both in traditional folklore and modern practice. Includes indexes of folk names, magical intentions, and planetary associations.

It Will Live Forever, Beverly R. Ortiz
Julia Parker, a Miwok/Paiute Indian, is famous for her beautiful baskets, but in this book she describes to Beverly Ortiz the ancient tradition of using acorns as food, from gathering to processing. This book is both interesting from a cultural perspective and useful as a resource to learn about acorn preparation.

The Foxfire Book, Eliot Wigginton
This and the rest of the books in the series are fascinating books on the traditions of the people of Appalachia, from moonshining to soapmaking to cabin building. Many of the books include accounts of how wild plants are used.

Fiction Books

The Earth's Children Series, Jean Auel
Five books (and a sixth on the way), starting with the acclaimed The Clan of the Cave Bear, tell the story of Ayla, a woman living in Ice Age Europe 30,000 years ago. Ayla is a skilled healer and uses many plants for food and medicine throughout the books. Auel clearly did a lot of research on plant uses.

Hatchet, Gary Paulsen
A 13 year old boy get stranded in the Canadian wilderness, and must fend for himself. A very enjoyable book, including mentions of plants that Brian eats, although he doesn't know the names (I believe Paulsen names them at the end of the book). There are several sequels which also mention plant foods.

Into the Forest, Jean Hegland
In the near future, a pair of recently orphaned teenage sisters are forced to support themselves after civilization collapses. Eventually they turn to the forest for food, and the narrator, Nell, becomes an expert on wild plants.

The Harry Potter books, J. K. Rowling
Throughout all the books, Harry and his friends make various potions using plant ingredients. Most of the uses are made up, but some are based on real folklore. Particularly interesting is the mandrake, which is important in the second book, Harry Potter and the Chamber of Secrets, when it is used to revive victims of petrification. Rowling made up that use, but most of the other characteristics she mentions are based on traditional beliefs.