.NET is thoroughly object oriented. Not only does .NET allow you to use objects, it demands it. Almost every ingredient you'll need to use to create a web application is, on some level, really a kind of object.
So how much do you need to know about object-oriented programming to write .NET pages? It depends on whether you want to follow existing examples and cut and paste code samples or have a deeper understanding of the way .NET works and gain more control. This book assumes that if you're willing to pick up a thousand-page book, then you're the type of programmer who excels by understanding how and why things work the way they do. It also assumes you're interested in some of the advanced ASP.NET