If you deal with Gangs at all from the patrol to investigative fields, this is your book.
Gangs are infulencing every aspect of society including the military...that's why the Navy's NCIS used this book as a reference!
It is highly informative and will give you the necessary training to start you out in this field and keep you going. Of course gangs change signs, tactics and methods as they see fit so certain portions will obviously be out of date but that is inevidable.
This manual should be purchased and read by patrol officers up to senior supervisors. Like that saying goes, the more you know the more you grow (and stay alive to beat the bad guys).