Welcome to the club Steve!

Best book is Graeme Wright's "Shooting the British Double Rifle" now in its second edition. Really the best there is.

Less good but with good eye candy is Alexander Gray's "The Hammerless Double Rifle,".

Also useful:

Craig Boddington, "Safari Rifles,"

Gregor Woods, "Rifles for Africa."

John Taylor, "African Rifles and Cartridges."

Two really good journals are the "Double Gun Journal" and "The Shooting Sportsman".
Also look for back issues by Ross Seyfried in the "Double Gun" Journal. Like vol. 5 issue 4 1994; vol. 6 issue 1 1995; vol. 10 issue 4 winter 1999.

Sherman Bell also has some fun stuff, see 15, issue 1 sprind 2004.

Also for general stuff on break action double barrels many of the better shotgun writers would be good to read for arguements on sidelocks vs. boxlocks, triggers etc.

I'd recomend almost anything by Michael McIntosh, in particular "Shotguns and Shooting," "More Shotguns and Shooting," and "Shotgun Technicana". Also anything by GT Garwood/Gough Thomas.

Just a start!