Use of let and into keyword, and how they help in making Progressive queries but still keep Defered execution.

into and let create temporary reference to store the result of a subquery/subexpression that can be later queried itself. These operators keep deferred execution and allow creation of sub routines that can be used incrementally or progressively. Personally, I would rather try simplifying the query, so I will not use either of these keywords. A few separate queries are much debugger [developer] friendly and less bug prone.

As a sample, let can be used to store the count of elements in a group

var categories = from p in products
	group p by p.Category into g
	let elCount = g.Count()
	select new 
	{ 	Category = g.Key, 
		ElementCount = elCount 
	};