APIs or databases that allows one to retrieve the works that a book cites Are there APIs or databases that allow to retrieve the works that a book cites. Eg. I give it an ISBN, and it returns all the works that it cites (not those it is cited by).

APIs or databases like Microsoft Academic Search usually only contain citations of journal articles and papers, but not books.
Mendeley has quite powerful database on all kind of sources. 

http://dev.mendeley.com/



Also, you should check Elsevier's developer portal, as they provide a number of APIs for their services:

http://dev.elsevier.com/

Where ScienceDirect API is especially relevant for you:
http://www.elsevier.com/solutions/sciencedirect/support/api

As you probably know, Elsevier is one of (if not the) the biggest publisher(s) of academic publications, including books. 

In case you might want to query your own instance of Mendeley Desktop - check my blog post, for few starting points:

http://blog.veles.rs/mendeley-desktop-http-api/