An Introduction to Database Systems C.J. Date ISBN: 0201385902
A textbook explaining the relational model in great detail, and written by a well-known relational database commentator. New editions of this book are regularly released every two or three years, so the ISBN might be out of date; the ISBN is for the seventh edition, but you should check if there's a newer one. (On the other hand, old editions are still perfectly readable and useful, and likely can be found much more cheaply.)
A textbook on database concepts (relational algebra, entity-relationship modelling), programming (SQL), implementation (files, indices, hashes, query optimization, transactions), and "advanced concepts" (distributed databases, data mining, object-databases). This book is an alternative to Date's book.
Managing Gigabytes: Compressing and Indexing Documents and Images Ian H. Witten, Alistair Moffat, Timothy C. Bell
1558605703, Morgan Kaufmann, May 1999.
Discusses algorithms and techniques for compressing, indexing, and searching large document archives. It's aimed at applications where you write your own disk storage and compression routines, and doesn't use SQL.
The Data Access Handbook: Achieving Optimal Database Application Performance and Scalability John Goodson and Robert A. Steward
ISBN: 0137143931 Prentice Hall, March 2009.
Addresses the issues of connecting languages to databases in a generic sense.