Jon, I have a copy of BENTLEY A Racing History, which is a Haynes publication. A large, quite heavy and detailed tomb with many images, the quality of some early ones is less than ideal, as if they may have been enlarged just a tad too much, but none the less all part of the history.
I guess the other Book that might be thought to be a prerequisite for a true Bentley enthusiast may be Full Throttle, the biography of Tim Birkin.
I suspect there will be leather bound copperplate top end books available, though the Haynes publication seems not to lack detail.
The English High Court Case of Vintage Bentley Old Number One, makes for interesting reading, and a transcript was available on GoMoG last time I looked... Perhaps providing an insight to the application of law to the concept of originality and the value thereof relative to vintage race cars and old machines in general...?
Good luck in your search Jon.
The ultimate (can you say the "Rolls Royce" of Bentley books ... no, of course not!) ... let's say the definitive work of reference is Clare Hay's "Bentley - The Vintage Years", which details the known history of every W.O. Bentley chassis. The 3rd and most recent edition costs £400! I long harboured an ambition to own a copy but even the previous editions fetch 3 figure sums when they turn up so I doubt I ever will. In the meantime, I content myself with regular browsing of
vintagebentleys.org which contains a lot of the same information.
Hay is the acknowledged marque guru and was an expert witness in the Brewer v Stanley Mann trial.
"Full Throttle" is a very entertaining read, at a much more accessible price. It was dramatised by the BBC some years ago with Rowan Atkinson as Birkin.