Is there a good book/article discussing the modularity in database design? By modularity I mean the keeping the tables belong to individual modules separte. Do not allow join between tables of different modules. And the kind of issues discussed in the question http://stackoverflow.com/questions/69128/saas-database-design-multiple-databases-split
This is a classic but good article going over a lot of the basic principles of relational database design. You will want to read up on topics like normalization in particular.
Relational database theory/design is a whole field of specialty, and there are tons of books on Amazon, college courses -- there are folks with Ph.D.s in this topic! I don't have any other personal recs, but I'm sure Google will turn up a lot of stuff.
Database System Concepts, a "classic" covering the theory from the basement up to the issues in concurrent access and design/implementation. It worked for me ;-)
The LinkedIn architecture slides that were posted on my original question were VERY helpful to me. It was cool to see how a real company progressed over time and changed their architecture to meet their needs.
I also liked a link someone left as well - http://highscalability.com/