akrange,

Welcome to the forum. A book not yet mentioned that I will suggest is The Hammerless Double Rifle by Alexander Gray. There you will find detailed explanations and pictures of the complex process of building a double rifle. It provides knowledge of the many components and parts of a double.

I would also like to emphasize reading the older posts on this forum. Some are filled with information you'll find nowhere else.

I hope this helps.

Curl