Wendell Santos has posted on behalf of ProgrammableWeb a list of fifty-three (53) book APIs!
Fairly good listing but it could be better.
For example, it is missing the Springer API, http://dev.springer.com/, and although they don’t list Elsevier and say http://www.programmableweb.com/api/elsevier-article is historical only, you should be aware that Elsevier does offer an extensive API at: http://www.developers.elsevier.com/cms/index (called SciVerse).
I am sure there are others. Any you would like to mention in particular?
Now that I think about it, guess who doesn’t have a public API?
Would you believe the ACM? Check out the ACM Digital Library and tell me if I am wrong.
Or for that matter, the IEEE. See CS Digital Library.
Maybe they don’t have anyone to build an API for them? Please write the ACM and/or IEEE offering your services at your usual rates.