In the Programming Microsoft LINQ book we dedicated two whole chapters (76 pages) about the writing of a IQueryable LINQ provider: one is about expression trees and the other covers the several ways to extend LINQ, including the writing of an IQueryable provider. I know that the subject is complex and probably is not necessary to every programmer. However, a good understanding of what happens under the cover of an IQueryable provider is good for everyone using any flavor of LINQ: when you debug your code, it might help you in finding issues faster.
I wrote this introduction just to explain why you should read this post of Bart De Smet, which is undoubtedly shorter than the corresponding chapter of our book and gives you a very good step-by-step introduction of the inner workings of an IQueryable LINQ provider. Then, if you really like this kind of things, you have another good reason to read the book :-)