I just send the last revision of my chapters for TK 70-561 to the editor. Yeah! Hopefully the book will go to print soon. You can pre-order already on Amazon!
For me the last chapter to work on was LINQ to SQL, but the book includes chapters on learning ADO.NET, Typed DataSets, LINQ to SQL and Entity Framework. You'll need to know it all.
Stephen Forte has an interesting post on the relevance of LINQ to SQL.