Rubio, M. (1998). Rattlesnake: Portrait of a predator. Washington, DC: Smithsonian Institution Press.
This book offers a perspective on the natural history and the anatomy of Crotalus and Sistrurus with good consideration to readers of all levels. The author's other profession is photography; plenty of beautiful pictures are found throughout the book.
Ernst, C. H. & Ernst, E. M. (2011). Venomous reptiles of the United States, Canada, and Northern Mexico: Crotalus. Baltimore, MD: The Johns Hopkins University Press.
This is a scholarly book; if scientific scrutiny is an important factor to your readership. This book offers the most up-to-date overview of scientific findings (academic journals and other scholarly publishing) in Crotalus species that engages in synthesis and analytical comparison of datasets, rather than merely offering a ‘laundry-list’ of secondary sources. My only regret is that the study area is geographically bound above the Northern hemisphere, leaving more than a handful of interesting animals unrecorded.
Klauber, L. M. (1982). Rattlesnakes: Their habitats, life histories, and influence on mankind. Berkeley, CA: University of California Press.
I'm actually reading this book right now. Klauber is a household name in Rattlesnakes, and it's not disappointing thus far to say the least. Everything from their bodily functions, behaviour, prey items, and reproduction. Additionally, a good portion of the book is dedicated to their relationship to humans including the effects of the bite, treatment of the bites, and how they're perceived/used in different cultures.
Gumprecht, A. et al. (2004). Asian Pitvipers. Berlin: Geitje Books.
Unfortunately it's out-of-print. If you have some (or a lot of) extra cash, you can even purchase a copy from amazon - $850 for used and $1,200 for a new copy. Although the study area is limited to Asia, I hear it's one of the best books on arboreal pitvipers. I've been looking for one at a reasonable price for a while, maybe you will have better luck.
I'm also looking forward to additional suggestions from other users.