I spent several weeks this winter documenting the internals of Microsoft’s ASP.NET 2.0 providers. You can view the results at http://msdn.microsoft.com/asp.net/downloads/providers/default.aspx?pull=/library/en-us/dnaspp/html/asp2prvdr01.asp. While you’re at it, you can also download the source code for many of Microsoft’s providers and get a look behind the scenes at how they work. (The published source isn’t perfectly identical to the real source, but it’s extremely close, and unlike the actual source code, the published source code can be compiled stand-alone.) It’s not often that the public gets to see Microsoft source code, so don’t miss this unique opportunity!
It’s a credit to the ASP.NET team that they were willing to jump through legal hoops to publish the source code so developers like you can write high-quality custom providers. With the source code in hand, it wouldn’t be difficult to port the SQL providers to Oracle, MySQL, and other databases.