If you are into front-end development, then this book is a good resource. This book explains how to improve your site performance (loading time) significantly. I bought this book back in 2009 and learned a lot from it. Although if you use framework, most of these optimization can be done from the framework, but I still think this book is a good read if you are into front-end development.
Also, all of the points covered in the book is mentioned in Yahoo Developer Network's Performance blog.[...]
There's also a youtube video titled "High Performance Websites and YSlow!" which explains these points in details.