If you look hard enough, they are available at not unreasonable cost. I got a copy of "Shooting the British Double Rifle" by Graeme Wright locally last year and recently found a copy of Gray's "Hammerless Double rifle" from a book seller in the US - I guess the weak dollar helps

There is a copy of Gray on ebay for $650 at the moment, but it doesn't look in such great condition. Anyway, its a hell of alot more than I paid for mine, so persistence pays off in the end!