Hi Pulsar & welcome to the forum.
I think this is what you are thinking of: Rail Guide 2017 (or other years) by Colin J Marsden.
https://www.amazon.co.uk/Rail-Guide-...lin+J.+Marsden
I've had a look through these before and they detail all the TOCS, including details of locomotives & rolling stock owned/operated by them, routes covered etc.
Hope this is useful.
Tony