Hi Justin
You are right, there are two books. Both out of print at the moment, but the complete allocations history reprint is off to the printers as we speak. I've got a revised version of the other book ready to go as well, so hopefully this will reappear in 2021.
The complete allocations book coming out this year has a list of all reallocations for every steam loco from 1948-1968. This is the book shown above .
The steam allocations book shows year start allocations 1948, 1951, 1954, 1957, 1960, 1963, and 1966 in a table format. It also contains maps showing the distribution of the engines of each class and a list of each shed's allocations on those dates. The new version of this I've been working on will now be using full colour to highlight regions.
So basically, the first book gives all the full details, while the second book provides overviews and different ways of looking at the data.
Hope that helps.
Hugh Longworth