This appendix gives a short list of suggestions for further reading. I apologize for the fact that the majority of the references are to publications for which I'm either the author or a coauthor....They're in chronological order by date of first publication.
Robert R. Stoll: Sets, Logic, and Axiomatic Theories. San Francisco, Calif.: W. H. Freeman and Company (1961).
The relational model is, of course, solidly founded on logic and set theory. This book provides a fairly formal but not too difficult introduction to these topics. See also the book by Hodges, later.
E. F. Codd: "Derivability, Redundancy, and Consistency of Relations Stored in Large Data Banks," IBM Research Report RJ599 (August 19th, 1969); "A Relational Model of Data for Large Shared Data Banks," CACM 13, No. 6 (June 1970).
The 1969 paper was Codd's very first paper on the relational model; essentially it's a preliminary version of the 1970 paper, with a few interesting differences. That 1970 paper—which was republished in "Milestones of Research," CACM 26, No. 1 (January 1982) and elsewhere—was the first widely available paper on the subject. It's usually credited with being the seminal paper in the field, though that characterization is a little unfair to its 1969 predecessor. In my strong opinion, every database professional should read one or both of these papers every year. Exercise: This latter sentence (not counting that initial "In my strong opinion," of course) is ...