James, as a quick, easily read guide/primer on better guns made in Europe, North America as well as GB, try Michael McIntosh's 'Best Guns'.
You could then move on to Christopher Austen's 'Modern Sporting Guns' for the British market. That could be closely followed by David Baker's 'Heyday of the Shotgun'.
Next up I would suggest both of Diggory Hadoke's books 'Vintage Guns for the Modern Shot' and 'The British Boxlock Gun & Rifle'.
Once you have absorbed that lot you will have probably started to become interested in certain makers, type of guns, aspects of the market and so forth and can build up a more specialised library. Anything by Gough Thomas, Gerald Burrard and GT Teesdale-Buckell will enhance your technical and historical knowledge.
As has already been pointed out, at some stage you will wish to have the three volume Crudgington & Baker masterpiece on 'The British Shotgun', as well as the admirable three volume Brown works on 'British Gunmakers', though they are better as reference books, I find.
Donald Dallas and Don Masters have published some excellent single maker books, some of which you may wish to obtain once you know which makers you are into.