Here's a book on them. If you don't want to buy, see if your library can get one fro you to borrow - they can borrow from other libraries elsewhere, and you get a longer time to keep the book then.
[ame="https://www.amazon.co.uk/History-Rover-Cycles-John-Pinkerton/dp/0950743194"]Amazon.co.uk: History of Rover Cycles: John Pinkerton, Derek Roberts: Books[/ame]