Manual of Vascular Plants of Northeastern United States and Adjacent Canada by Henry Allan A. Gleason and Arthur Cronquist
If you are going to be a serious botanist in the Northeast, you need this.

Illustrated Companion to Gleason and Cronquist's Manual by Noel H. Holmgren
This contains pictures of every plant in the above manual. Expensive, but worth it. Too heavy for the field. The binding of those copies I've used is poorly made.

Spring Flora of Wisconsin by Norman C. Carter Fassett
Nice technical manual. The problem is: what about summer?

Fruit Key and Twig Key to Trees and Shrubs by William M. Harlow
Essential for winter botany and very affordable.

Newcomb's Wildflower Guide by Lawrence Newcomb
A very simple illustrated plant guide with a key. Good for beginners. Out of date though.

Field Guide to Wildflowers of Northeastern, North-Central America by Roger Tory Peterson
I prefer this to Newcomb's mainly because it has arrows pointing to the key characteristics. But it doesn't have a key. Out of date too.

Field Guide to Eastern Trees by George A. Petrides
The easiest eastern tree book I've found.

Edible Wild Plants of Eastern and Central North America by Lee Peterson
If you like eating things you find in the woods, try this.

Ferns by Boughton Cobb
The easiest fern book I've found.

Flora of the Great Plains by The Great Plains Flora Association
The technical manual for the Great Plains. Good, but out of date.

Grassland Plants of South Dakota and the Northern Great Plains by James R. Johnson and Gary E. Larson
The best picture book for that area that I've found.

The Jepson Manual: Higher Plants of California by Willis Linn Jepson
The most up to date and easy to use manual for California.

The Jepson Desert Manual: Vascular Plants of Southeastern California edited by Bruce G. Baldwin and others
Based on the Jepson Manual. It is an updated mini Jepson focused on the Deserts of Southeastern California. Good for Southern Nevada and Western Arizona as well.

Shrubs and Trees of the Southwest Deserts by Jan Bowers and Brian Wignall
This and the following two are pretty nice picture books. The pages are on colored paper corresponding to the flower color.

Shrubs and Trees of the Southwest Uplands by Francis Elmore and Jeanne Janish

Flowers and Shrubs of the Mojave Desert by Jan Bowers and Brian Wignall

Pacific States Wildflowers by Theodore F. Niehaus
Easy, but extremely lacking. There are too many plants in this area to do a decent easy field guide for the entire area. Good for the very basics.

Southwest and Texas Wildflowers by Theodore F. Niehaus, Virginia Savage, and Charles L. Ripper
In the areas I've used this I needed to use it with the Pacific States Wildflowers guide. Together they worked okay for the Utah, Nevada, Arizona area.

Field Guide to Western Trees by George A. Petrides
The easiest western trees book I've found.