We are almost complete with part 2 of the Provider Toolkit (in editing now to be published in a few weeks) which addsa nother 70+ pages of content on providers (specifically the MSFT ones, Windows AD/AZMan, SQL Server, etc.).  In due time,we'll also be releasing the source code to to give you tons of insight. 

In the mean time, here is a great article about a SQL SiteMap provider, so you can store the site structure in the database instead of in the web.sitemap file.  Jeff Prosise at Wintellect has been writing our provider toolkit for us, so he is definitely one of the foremost authorities on the subject.  Enjoy!