"A Relational Model of Data for Large Shared Data Banks," by E. F. Codd shows that the relational model was first conceived by E. F. Codd. This is the paper that started the entire relational database industry. Originally published in 1970 in CACM 13, No. 6, this paper is now available online as a PDF file through the ACM Digital Library:
If youd like to read up on the roots of the relational model (set theory and predicate logic), a pretty readable math book is Discrete Mathematics by Richard Johnsonbaugh (ISBN: 0-13-089008-1). We have found it to be succinct on the topics of set theory and predicate logic.
An Introduction to Database Systems by C.J. Date (ISBN: 0-32-119784-4) is a classic overview of database systems with an emphasis on relational database systems. You should consider it essential reading if you want to know your craft well.