A classic by Fritz Onion (from Pluralsight). Recognised as a .net expert this a book for people who are interested in knowing more than just practical application, and are interested in the framework itself.
Recommended by a colleague, I purchased this in combination with the more recent second edition Essential ASP.Net 2.0 (Microsoft .Net Development)
, which claims to supplement instead of replace this first edition.
Only knowing MVC I found this book useful for understanding legacy WebForms applications and worth the low price secondhand.