Lately, most information of any value can be gotten from the Web. But the Web was pretty wimpy 15 years ago when I started this project, so all my original information came from books, listed below by frequency of use in preparing this catalog. And of course, you can't carry the Web with you out in the woods (well, I suppose you can do that now, too). But in any case, I still find field guides in the form of books much easier to use for identifying plants, than anything I find on the web (that is, except for my own identifier).
This is far and away the best field guide to wildflowers in the world, or at least in the U.S. It has a near-flawless system for identification based on things you can easily observe: number of petals, shape and arrangement of leaves, etc., that, when combined with the excellent line drawings, invariably get you to exactly the plant youre looking for. This system would not work if not for the fact that the book also is also so complete -- 1375 species including flowering vines, small trees and shrubs. While there are probably many more species if you include all the rare ones and near-indistinguishable relatives, you would be very hard pressed to find any in the field that arent in this book.
To remain compact and easy to carry, this book is purely an identification guide and provides little additional information about any plant. Also, unfortunately, it only covers the northeastern U.S., and there is no book even close to this in usefulness for other parts of the country.
The descriptions, drawings, and coverage would make this book a close second to Newcombs, except for the near-total lack of order to the groupings of flowers. It is divided into main sections by color, but beyond that you have to leaf through every page reading the titles to see whether your specimen would belong on that page. Even this would not be so bad except that there is no table of contents of the page titles. It does get easier to use once you are used to it, but it can be tough for a beginner. Nevertheless, I find this book very useful in cases where Newcombs description leaves a questionable identification, as the descriptions and drawings are very good, and it often points out different distinguishing characteristics.
Despite these flaws, the textual descriptions are wonderful, adding a great deal of information about each plant, not just for identification purposes, but interesting facts like where it came from, its uses, how it was named, etc. Because of this I never consider an identification complete without consulting this book. Also there are quite a few more descriptions than photos, and the photos arent entrely useless, as they show you a real plant rather than a somewhat idealized drawing that tries to highlight more characteristics than you might see in any one specimen.
Also, while this book leaves out trees and many flowering shrubs, it contains seeveral grasses and sedges that are not in the other references.
Aside from having possibly the longest title in the world, this is possibly the easiest-to-use field guide to woody plants (trees and shrubs). It has a system of identification that involves twigs, leaves and buds, so is useless for narrowing down a specimen based on a flower (and in fact, often completely ignores the flower), but this system works whether or not the specimen is in bloom. Still, it is not always easy to identify a tree or shrub, especially if you can't find a good bud to count its scales. My wildflower catalog does not include large trees, but it does list shrubs and small trees that have noticeable flowers.
This is a classic, steeped in cryptic abbreviations that take a while to learn, but it seems more complete than the Petrides Field Guide guide above. It has been essential for identifying several local species listed nowhere else.
This is a massive 1000-page color glossy book of over 10,000 plants (including trees, shrubs, flowers) that can conceivably be used in a garden, and how to plant them. It is poor as an identification guide, and doesnt list all insignificant weeds that nobody in their right mind would plant (but curiously, it includes Poison Ivy). But it has helped me identify some plants that appear to be growing wild, which I could not find in any wildflower books. It has so many exotic plants it is also fun to read and look at the pictures.