To the best of my knowledge, there are no English language "books" on caecilians. There ARE several largely taxonomic monographs, with Taylor's including what natural history data as was available:
Ronald A. Nussbaum & Mark Wilkinson, 1989. On the Classification and Phylogeny of Caecilians (Amphibia: Gymnophiona), a Critical Review. Herpetological Monographs, Vol. 3. (1989), pp. 1-42.
http://www.bmnh.org/PDFs/RAN_89_mono.pdf
Edward H Taylor, 1968. The Caecilian Species of the World. University of Kansas Press, Lawrence.
R S Pillai and M S Ravichandran, 2005. Gymnophiona (Amphibia) of India a Taxonomic Study. Zoological Survey of India, Zoological Survey of India : Occasional Paper No. 172, Reprint, vi, 126 p, fig, maps.
Gymnophiona (Amphibia) of India a Taxonomic Study/R.S. Pillai and M.S. Ravichandran
David J Gower & Mark Wilkinson, 2005. Conservation Biology of Caecilian Amphibians. Conservation Biology 19(1): 45-55.
http://www.bmnh.org/PDFs/DG_05_CB.PDF
Gopalakrishna Bhatta, 1998. A field guide to the caecilians of the Western Ghats, India.Journal of Biosciences, Vol. 23(1): 73-85
http://www.ias.ac.in/jbiosci/march1998/JB-1K.pdf
Because those are largely taxonomic works, it's also important to have the most recent treatment of taxonomy:
Mark Wilkinson, Diego San Mauro, Emma Sherratt and David J. Gower, 2011.
A nine-family classification of caecilians (Amphibia: Gymnophiona). Zootaxa 2874: 41-64.
A nine-family classification of caecilians (Amphibia: Gymnophiona) (Emma Sherratt) - Academia.edu_
For good data, you're going to have to dig for scientific papers dealing with regional herpetofauna and with particular caecilian species, plus the occasional forum threads that contribute specific, useful data. Then compile it, reference it properly, obtain images and such...and submit to a publisher. Voila, the book you are seeking!
