You can find here some books and journal, that may help you or you can find useful information there.
There are three parts on this page - books about Haworthia and Gasterias, journals and other books about succulents...
Part I. - books about Haworthia and Gasterias
Essays on Haworthia - Volume 2 by Bruce Bayer 2006
Essays on Haworthia - Volume 1 by Bruce Bayer 2002
The New Haworthia Handbook by Bruce Bayer 1982
Haworthia Handbook by Bruce Bayer 1976
The World of Haworthias, Volume 1 by Ingo Breuer - Bibliography and annotated index, 1998
Notes on Haworthias by J.R. Brown, Compiled by Stephen Halloway 1999
The First 50 Haworthias by John W. Pilbeam 1970 and the Second 50 Haworthias by John W. Pilbeam 1975
Beautiful succulents Haworthia by Takashi Fukuya, 2006 - perfect photos...
Gasterias of South Africa, A new revision of a major succulent group by E. J. van Jaarsveld, Illistrated by Ellaphie Ward-Hilhorst, 1994.
Part II. - journals