There is no perfect book.
I would though, recommend the following :
BR Equipment and BR Equipment 2 by David Gibbons
Electric Traction Systems and Equipment by D.W. Hinde
Diesel Engines and Transmission Systems by D.W. Hinde
An Introduction to Diesel-Electric Traction Equipment by The English Electric Company
They don't really cover new stuff but will give you a lot of information about the BREL stock and older locos we still have running.
Other useful resources are on the web.
http://members.madasafish.com/~dysgraphyk/156/ is a really good resource on the Class 156 but a lot of the tech applies to the 15x and 16x fleets as a whole. Look at the Under the floor area on the site as an example.
https://www.steane.com/index.php/technical-documents-archive from the website of Paul Steane, an engineer whom I highly respect and the documents span his long career.
The books that Simon Lilley has written normally have some good technical detail, in particular the book on the Class 71/74 locos. I also have good books on the 33s and the Peaks by the same author.
If you find any others let me know, you can't have enough books or not need to learn more!
