When (many years ago) I wanted to get a handle on "modern" diesel locos I got myself this book.
British Rail Main Line Diesel Locomotives
Colin J. Marsden, Graham B. Fenn
ISBN 10: 0860933180 / ISBN 13: 9780860933182
Published by OPC Railprint, 1988
Now, good as it is, there are two problems.
1) The book is only up to date as far as class 60.
2) It's out of print, so go online and search for a used copy.Try Amazon.
or,
https://www.abebooks.co.uk/British-R...BoCJK4QAvD_BwE