Relational Databases

Codd, E.F. The Relational Model for Database Management : Version 2. Addison-Wesley, 1990 (written by the originator of the relational model, this volume covers the practical aspects of the design of relational databases).

Date, C. J. An Introduction to Database Systems, 6th Edition. Addison-Wesley, 1994 (a classic book, revised--the best, most comprehensive, and most up-to-date treatment of database concepts and technology available).

Ullman, Jeffrey D. Principles of Database & Knowledge-Base Systems Vol. 1, Second Edition. W.H. Freeman & Co., 1988 (the classic text on relational database theory).