Between six and eight quid each so far. Depends on the class, really. Every class member is listed, as well as some useful photos. There is also a weight diagram at the back.
I've picked up "S", "V", and "WT" so far. Waiting for "Q" and "800". Not sure the author will touch the 101/J15s - there are too many of those and, anyway, a very good book "Steam In Three Centuries" has covered that class.