The book you choose depends a lot on what info you seek.

If you are primarily interested in how DRs are built, Gray's book is good. Also on that topic there are books about double shotgun production with great crossover information. Try F.LLi Piotti by Marco Nobili. Piotti makes their shotguns just like the Brits did in the years before WW2 - no CNC, just hand machines and hand work. This book details the progress of production from raw steel and wood to the final work of art, with a lot of photos along the way. I know of no book with such a clear exposition of hand-made double gun production.

If you are interested in classification of makers and dates of production you should get the two volume set British Gunmakers by Nigel Brown.

There are various works about specific houses of gunmaking such as Purdey, H&H, etc. if your interests lie with specific makers.

I recently read Shotgun Technicana by Michael McIntosh and David Trevallion. There is some great information there, and a lot of crossover to DRs.

I really don't know of a work that covers all areas of interest.

Curl