No doubt, this is the bible of OSPF, the ultimate book on the subject matter.
J Moy has an amazing writing style and this book is a page turner. It starts from the fundamentals of link state algorithm and then moving onto more specific OSPF message types to OSPF areas etc. It also covers Multicast OSPF and OSPF MIB.
There is a really nice FAQ section that answers your day to day OSPF questions and comes as a handy reference.
Who better to write the book on OSPF than the person who wrote the RFC and worked hard to make OSPF the dominant IGP! and he also happens to be one of the guys at Sycamore.