Migrating and need database assistance

I have quite successfully migrated three sites now theme and all to Drupal. Everything is going wonderfully, but I am hung up on one thing. Several pages over the three sites use some database connectivity to retrieve and set some data based on what the user is doing. It all seems trivial and straight forward. I was using adodb and really liked it. But is there documentation somewhere and some examples of what is available to me via Drupal code for accessing the database? Queries on drupal.org tend to come up with a lot of module hits and people talking about database tuning and what not. What I need is some nice documentation on what I have available to me.