The Plant Book: The world of Plants in a single volume was first published in 1997 as "Botanica," it then went through several revisions in 1998 and 1999, reprinted in 2000 and then revised under the present name and published in 2001.
I "lucked" into this book (and it truely was lucky) on a bookstore "closeout" shelf in 2002 where I was able to purchase it for $18 (it originally sold for $100). This book is a real "tome" at 1020 pages and carries a wealth of information about just about any plant known. It covers all plants and trees, garden, wild, exotic and tropicals as well. So far I haven't run across a plant I couldn't find in this book.
It carries a hardiness zone map, major plant groups, and an alphabetized A to Z of plants with interesting facts about the Genus, geographic origin, cultivation, propagation and synonyms. Under the entry listed for each species it gives information about height and spread, variants, hardiness, and general information.
It also has a table of plant families, and index of common names and synonyms. The whole book is full of wonderful photographs, and while it might fall short within a specific genus of listing all the cultivars (like hydrangea for instance), it still covers more ground than some other encyclopedias and even specific plant books that I've seen.
I find this the most used resource I own for looking up plants, and heartily recommend it if you are able to find it at a used bookstore!