Since you're here, I'm guessing you already know this one, but just to be sure:
SQL for web nerds. Just anything residing in the
/books/ directory, really.
Apart from that, I'd like to recommend The Sql Guide to Oracle.
It is bit old (February 1992), but I find it to be good for beginners who need to get a hold of SQL programming. It starts from very basics and takes you to pretty high level. Despite the "Oracle" in the title, almost none of it is Oracle-specific.
For more OpenACS4-specific data modelling advice, I'd first read through the (now outdated) docs on datamodels and the object system, fire up your favorite editor, read some datamodels from OpenACS 4 and try to make the connection. The need for documention on OpenACS 4 is bad.
Oh, and look at the bookshelf, too. Are the any other people out there maintaining bookshelves on OpenACS/ACS matters?
