I was very happy to get a chance to look through my copy of Essential LINQ today.   While there are a number of books out there on LINQ, this one from Dinesh Kulkarni and Charlie Calvert is a must read!  They do an excellent job of explaining the core philosophy behind LINQ as well as its practice in a number of different domains including Linq to Sql, Linq to Entities, Linq patterns, etc.  Dinesh and Charlie were both intimately involved in the design of LINQ from the very early days.  Dinesh now leads our efforts on .NET RIA Services where we continue to carry the LINQ vision to its logical next steps  and Charlie continues to keep be an expert in listening to the C# and LINQ community! 

Essential LINQ