Published in 1948, this was intended as a line-side spotters guide, giving detailed information how to tell your Gresley V-1s from your V-3s, Castles from Kings as they shot past. It gives a history of the Big Four companies, a table of silhouettes in the front and much technical detail about each class covered, which was pretty much everything the average enthusiast was likely to see when the book was published.
Did you know that all classes of engines on the LNER prefixed with a "B" were 4-6-0s? That the LNER had a pair of streamlined 4-6-0s which looked just like mini-A4s? That the mighty Great Western Kings were so heavy that their cabside route marking was "double red", meaning that they had restricted route availability even on the mainline? If you do, you probably don't need this book. If not, it's a mine if useful information!
I've got a copy of this myself and rate it highly. Front cover slightly loose, otherwise in good condition.