It's packed full of interesting info and good ideas. It is a cracking read, too, and pretty funny in places (especially some of the smart-arse captions to the period prints). Worth having just to read even if you aren't going to play it. To be clear though, it isn't a "supplement" like Chick said but a full-blown game system. You don't have to buy anything else to play it.
Glad I got mine from Spirit Games when it was still 25 quid.