There are any number of basic guides to reloading, however one of your countrymen will have to advise on the most logical current offering stateside. In Australia the recommendation would be the latest edition of "Nick Harvey's Practical Reloading Manual".

In addition to the basic guide, you will also need Graeme Wright's invaluable book "Shooting the British Double Rifle". It assumes you already know the basics, and moves on to the nitro and BP double rifle cartridges.

Hope this helps.