EDIT: My initial review criticized this book's lack of useful information for making markers, etc, but really this was a misunderstanding of OpenLayers on my part. This book has in-depth information about how to build "features" on vector layers, which are many times more powerful than "markers". There is also lots of information and reference material in this book about events and event listeners, which is sometimes hard to find in the online documentation.
I have increased my rating of this book to 5 stars because of how useful it has been to me. I started building a map using this book when I knew nothing about OpenLayers, and have advanced to making custom controls, complex events, and advanced styling based on the information in this book!