I personally recommend the "
The Firebird Book: a Reference for Database Developers" By Helen Borrie.

I myself have this (along with the supplements), and i have found it very useful while developing database oriented software with Firebird.

About Relation databases in general, there are a lot of information on the internet about it, sites, papers, ebooks, you name it! A book that might give you some insight about RDBMS ANSI SQL is "Teach Yourself SQL in 21 days", its a simple book but it might give you some interesting concepts to go on!

Luis Semedo Duarte