APIs or databases that allows one to retrieve the works that a book cites
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/