The two I use the most are Peterson's Field Guide to Wildflowers and Newcomb's Wildflower Guide. The first has plants grouped by color of bloom and the second by botanical relationships and uses a key to narrow the plant down if you are starting blind. Both have their fans. If you want to learn grass-like plants (God help you) you'll need a more specfic guide to those plants, as you would for ferns or woody plants. If you are going to mostly be in our area, Howard Boyd wrote two field guides for the Pine Barrens. One covers all groups of plants and animals, although it doesn't include everything in each group you might run into. The other just covers just wildflowers, and groups them by bloom schedule. I find both valuable.
Eventually, you'll be lugging around a library, as I do. But the one book you would have found most usefull will always be sitting at home on the toilet tank