Gentians of the Eastern United States
The family Gentianaceae includes Sabatia, Frasera, Bartonia, Obolaria, Centaurium, Eustoma, Halenia, Lomatogonium and Voyria, as well as Gentiana, Gentianella, and Gentianopsis. It's the “blue-flowered gentians” that come to mind most readily, though, and identification of these to species requires careful attention to small details which most field guides don't have space to illustrate or discuss more than briefly.
Jim Drake, president of the Georgia Botanical Society, sensed this need and set out on a multi-year quest to compile a photographic family album which culminated in the book Gentians of the Eastern United States. The book includes detailed descriptions, photographs and drawings of each species within the family, along with notes on the features that distinguish them from similar species.