The book that started it all: the original Field Guide to the Birds
Audubon's field guides use pictures instead of drawings
When you're interested in nature, as I am, you often spend a lot of time out in it, on nature walks and suchlike. When you do, you usually want some kind of reference to tell you what it is you're seeing. That was the idea behind Roger Tory Peterson's Field Guide Series, and where Peterson went others followed. Now, my library includes a whole shelf of field guides.