As you may have noticed, the second volume is out, which deals with the JF6, PL2, YJ and SY Classes.
Work is in hand on the third volume, on the QJ Class, which seems to be the one everyone wants. This will be a fatter volume than the first two as there were so many locos in the class. Unlikely to be published before 2017 and it would be nice to sell more of the first two volumes to get a bit nearer breaking even.
Volume 4 will cover the Pacifics, the FDs and a few other of the classes that lasted into the enthusiast era.