The first reason is that it is beautiful. It is a complete translation of a book on Shuijiao from the 1930's with grainy black and white photos that are easy to see and very detailed. (No editing necessary! Thanks North Atlantic Books.)
The second reason is that it is material that helps to put internal arts in context. Shuijiao is a kind of gentrified form of Mongolian wrestling, stand up throws, with similarities to Judo take downs and Sumo too.
Shuijiao isn't however all that gentrified, (push-hands should get the gentrified wrestling award). Shuijiao throws are elegant, they make great police training for dealing with drunks, which historically is the biggest part of a policeman's job.
Practitioners of internal arts will quickly see how shuijiao techniques are a part of every taijiquan and baguazhang movement. It is kind of a mini-subject within a subject.
Go get it!