I would very highly suggest getting a copy of "Shooting the British Double Rifle" by Graeme Wright.

Midway has 1 copy available for 40 bucks;

http://www.midwayusa.com/eproductpage.exe/showproduct?saleitemid=733496

Also "The Hammerless Double Rifle" by Alexander Gray. This book is somewhat rare so prices have been going up recently.

Both excellent resources for someone wanting to learn about doubles.