For implementation of database systems, Gray and Reuter's Transaction Processing: Concepts and Techniques http://www.amazon.com/Transaction-Processing-Concepts-Techni.... For the ideas behind relational databases, I like C.J. Date's Introduction to Database Systems, http://www.amazon.com/Introduction-Database-Systems-C-Date/d....

I recommend two authors, listed below with Amazon links to their most famous books:

C. J. Date http://www.amazon.com/Introduction-Database-Systems-C-Date/d...

Joe Celko http://www.amazon.com/Joe-Celkos-SQL-Smarties-Programming/dp...

It shouldn't be pricey. I got a copy of the 7th edition hardcover for about $8. This one: http://www.amazon.com/Introduction-Database-Systems-C-Date/d...

I doubt the concepts and mathematical model behind relational databases have changed that much between 1999 and 2003. :)

Also, most the code in the book is in a language called "Tutorial D", with asides about how it would look in SQL (and equally frequent complaints about how SQL fails to live up to the mathematical ideal of the relational model). It's not an SQL tutorial, but once you understand the relational model, you will probably be able to burn through tutorials -- it will just be learning syntax, not completely new ideas.