A great book that many pros use is: American Horticultural Society A-Z Encyclopedia of Garden Plants

I'd also recommend the series of books by Roger Phillips and Martyn Rix - 2 volumes on perennials, one on annuals and biennials. Lots of good color pictures, including different varieties, and for many plants, a picture of the plant in its native landscape. For me, that's almost the most useful aspect, because it tells me far more than the usual brief description about what conditions and companions the plant likes. (There are separate volumes on Roses and Shrubs, as well.)

Here is a link that might be useful: a great alpine nursery, but also a super list of books