Your question has a broad answer. There are probably thousands of commands.
I know a fraction of them, and I can perform most any task I need in my Access
ADP applications. What are you looking to accomplish?
My favorite book is Inside SQL Server 2000 by Kalen Delaney. I don't find the
exhaustive help system to be user-friendly enough for what you're looking for,
an overview and/or quick reference.