[dmfay]

Lex de Haan and Toon Koppelaars, Applied Mathematics for Database Professionals -- a fairly gentle introduction to database theory starting with logic & discrete math

Ralph Kimball and Margy Ross, The Data Warehouse Toolkit -- even if you're in an OLTP context, it's useful to know about warehousing techniques

Dmitri Fontaine, The Art of PostgreSQL -- cookbook approach to modern SQL with plenty of explanations

Not a book but Heinz Klein and Kalle Lyytinen's paper "Towards a New Understanding of Data Modeling" in the Software Development and Reality Construction remains a must-read 30 years later. The organizers of that conference have published a free PDF copy of SDRC if you look around a bit.