The distribution of species have mostly been sought in ‘The Genus Epimedium and other Herbaceous Berberidaceae including the Genus Podophyllum’ by William T. Stearn. Also this work has helped with the relationships between the species and some of their physical features.
The internet has assisted with some of the more obscure, post-Stearn species for verification of species status.
Whilst I haven’t used it as a reference for this website, I would thoroughly recommend Sally Gregson’s recently published book 'The Plant Lovers Guide to Epimediums'. It is a very readable book compared with the serious work by Stearn. It is very well illustrated and includes a number of my pictures , some of which you may find again in these web pages.