The single best book for double rifles is G. Wright's "Shooting the British DOuble Rifle". Great book for how they work, and getting old British rifles to shoot. The only "draw back" is that there is nothing on "modern doubles" whether Searcy or European.

Grey's book "The Hammerless Double Rifle" is good eye candy, with a good deal on double rifle construction, but it has some limits, and can be hard to find.

Boddington's "Safari Rifles," Woods, "Rifles for Africa", and of course Taylor's books have some info.

Try to look up back issues of Double Gun Journal with articles by Ross Seyfried and Sherman Bell, they have some useful stuff, with fun pictures