When I first got this book I thought it was going to be a compilation of tips and tricks. I was quite surprised with the fact that it really wasn't. The author starts out by telling you how you to determine the bottlenecks in your site then goes into a lot of samples on how to correct and / or improve them. He covers memory, CPU, caching, compression and more.
This book is definitely worth a read then a reread every once in a while. I plan on implementing some of the suggestions for my sites.
Check out the book on the Packt Publishing site or ASP.NET Site Performance Secrets .