There are several excellent gardening books out there but many of them are geared to a specif location. We like Neil Sperry's Complete Guide to Texas Gardening and Reader's Digest North American Wildlife, an illustrated guide to 2,000 Plants and Animals. With this last one we have only find a few plants that are not listed but this is general a wildflower book. If you go to ebay and put in "plant encyclopedia" you will get several like: American Horticultural Society A to Z Encyclopedia of Garden Plants, but most of these are just one volume.
As far as the internet, I have found Burpee's to be a great place for many plants. I don't buy their plants or seed just read about them. Most seed companies have good websites. Also Dave's Garden is fairly good.
For wildflowers I mostly use:
http://www.wildflower.org/plants/
You could print out information you find about the plants you are interested in and put them in page pockets in a loose leaf notebook.