First of all, I really like the book, I am (slowy) working my way through the bok, and I really enjoy the fact that you show both the IEnumerable<T> extension methods signatures, together with the query syntax. Too many books just focus on the query syntax in my opinion.
in Listing 4-10, the type of the result is "IEnumerable<Order> orders", which is not correct, since we are performing a "select new {..}", which creates an anonymous type. So, you should probably use "var order" as the type of the result.
Bennie Haelen (http://footheory.com/blogs/bennie/default.aspx)