If you want 'technical' as in the details of the patents etc, the British Shotgun Volumes 1, 2 and 3 by Ian Crudgington and David Baker is unbeatable. Still widely available.
There are various beautifully illustrated books (by Donald Dallas in particular, but also others) that are 'maker specific'.
There are a variety of older 'classics' like anything by Gough Thomas, Burrard, J.H Walsh, Greener etc. Many are available as reprints.
I would also recommend the Dig Hadoke and Nigel Brown books mentioned in previous posts