There is only one book on Hellebores(A Gardener's Guide to Growing)generally available. Written by Graham Rice & Elizabeth Strangman, Hardcover, Publisher- Timber Press, Sept. 1, 2001 ISBN 0881925179 Available from Amazon.com and others. A second book by Graham Rice, Hellebores, Paperback, Publisher-Cassell, Oct. i, 2002, ISBN 0304362875 is sometimes available on special order. Have not seen this book, nor any reviews. The Gardeners Guide is a good source of information about Hellebores and their cultivation, with many color photos. Written with a European(UK) point of view.
SB

The Rice and Strangman book has some problems and the photos are rather commercial-minded, but it is still a useful introduction where little else is available.
Another book is forthcoming from Timber Press (Judith Tyler and Cole Burrell) and may be available by the end of the year. Hopefully it will offer updated and more accurate information.
Occasionally you can find some other good ones that are out of print, but it isn't easy and can be expensive.
You can easily find the Rice and Strangman book for good prices, new or used.