Technical manuals - general
The Railwayman's Diesel Manual
William F. Bolton

1956 onwards

There are several editions of these book, the earlier ones have more DMU content, as there were less locos around!

Diesel Rail-Cars
(An Introduction)

R. H. Mann
Draughtsmen's And Allied Technicians' Association

Along the lines of the above, quite technical in places (some big equations), but a good look at DMUs. From the 1963-4 session.

Epicyclic Gearing
Major W. G. Wilson

The Institute of Automobile Engineers

A 1932 paper on the 'Wilson' gearbox, this is a 1956 reprint issued on training courses at Self Changing Gears Ltd.