X-windows manual/hints

[Braddock John Hathaway]

As some of you may recall from an earlier post on these two bboards,
I'm doing a project for an information systems course that deals
with graphics on UNIX systems.  I have just received word from my
professor that we can indeed assume that each system that my product
must be able to port to will already have X-windows.  In light of
this revelation, I have a question:

Does anybody out there have advice about what book(s) I should go out
and buy (or borrow from our library) regarding interfacing with the
X-window system from a C program?  Does anybody have any advice period?

I'm SUPPOSED to start prototyping soon.  I'd appreciate any help I
can get.

--Brad