I am currently using Subsonic by Rob Conery for an Asp.Net project and loving the fact that I do not have to write a line of Data Access code. I edit my Database tables, build in Vs2005 and go. Subsonic takes some of the tenets of Ruby on Rails and mixes it with what is good in Vs2005.
Rob has published some documentation on Subsonic. Its not exactly a manual but enough information to provide guidance on the conventions of Subsonic.
There is loads more information at the Codeplex site. If you are doing ASP.NET 2.0 Development and need to get a few steps ahead early in your development cycle then check it out.