I don't think there is anything that covers the whole south American continent.
I just bought Gunther Kohler's Reptiles of Central America and it is a pretty good book. If you want something just for CA, it is probably the best thing available that covers the whole area, partcularly the areas of Nicaragua and El Salvador (there are other good books for the Yucatan and Guatemala as well as the Costa Rican books).
Of course Jay Savage's Amphibians and Reptiles of Costa Rica is also excellent, but covers a much smaller area of Central America.
For South America, I don't believe any such book exists. There are books that cover the venomomous snakes of South America and books that cover particular regions, but the whole continent is not available in book form as of yet.
There was a little TFH book published in 1982 with a few pictures and a basic list of the snake species. I bought it from a petstore in the early 80s. Now it sells online used for something like $70, if you can find it.
Are you really going to be on the whole continent? Surely you can narrow down your areas and find more specific resources.
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Chris Harrison
Central Texas