As far as "shooting" versus "gun" books, it's hard to beat Brister's classic SHOTGUNNING: THE ART AND THE SCIENCE for a combination of both guns and shooting information. But aside from the authors Battue mentioned, British author Michael Yardley has also written some very good stuff on shooting technique, since he's a long-time Sporting Clays instructor/coach, including a book. I shot with Mike some years ago on a SC course in California, and he's very good at practicing what he preaches.
That said, shooting clays is somewhat different than shooting birds. Among the greatest "teachers" of bird-shooting technique is a trip to some place like Argentina, where the bird shooting is essentially unlimited. As I noted earlier, my first trip to Argentina was made with Bob Brister (and Grits Gresham!) in 1996. I'd already shot clays with Bob, with of course some suggestions on his part, but learned far more down there.
As he noted, you can stand in one place shooting the same basic angles for an hour or two, until you absolutely get them down--because you won't be able to load your shotgun fast enough to keep up with the birds going by. That sort of thing helps far more with gamebird shooting than any amount of clay-bird shooting. Of course, it also costs more than clays!
Have since not only been back to Argentina but also did some similar shooting in South Africa. RSA has an even larger variety of birds, and the same essentially unlimited shooting, and some of their birds are even more difficult targets. One of the guys I shot with in Africa claimed he averaged 90% on Argentine doves--and only hit one of the African Rock Pigeons with his first box of shells. They fly far faster than they appear to be, and are even more erratic in flight than any doves I've hunted. Anybody who does 30% on them is a good shot!
