The Theory of Relational Databases by David Maier.
From the webpage:
This text has been long out of print, but I still get requests for it. The copyright has reverted to me, and you have permission to reproduce it for personal or academic use, but not for-profit purposed. Please include “Copyright 1983 David Maier, used with permission” on anything you distribute.
Out of date, 1983, if you are looking for the latest work but not if you are interested in where we have been. Sometimes the later is more important than the former.
Enjoy!