It depends on where you live, of course, but I really like Stan Tekiela’s (or maybe switch the i and e) Wildflowers of __(your_state)___. Also Birds of…, Trees of… and I think there are some others. But the flowers and birds version were so helpful to me and my kids liked them too. They are arranged by color, and the birds book inludes an inset photo of the male/female if a different color. The books are a great size with a 2-page spread for each listing (left page a full color photo, right side with descriptions, fun fact, little map of your state to show where they are mainly found, etc.)
I tried other books, but sometimes they just got too overwhelming. These were very manageable for me to actually FIND what I wanted very quickly.