Not exactly on database model book, but Eric Evans's
Domain Driven Design is a great reference for domain modelling -> O/R Mapping. Scott Ambler's essays (google for his name) are probably the best general references on O/R mapping around. Fowler's most recent book (
Patterns of Enterprise stuff) gives a nice toy-level look at O/R mapping and design strategies, but should be treated as an introductory primer in that regard rather than a good way to do it.