There is a new paperback version of Nik Borrow and Ron Demey's Birds of Western Africa out now, published by Helm I think. The full hardback version is also at a reduced price on www.nhbs.com at the moment.
When I was working in Nigeria not that long ago I had a book called, I think, - Birds Of West Africa and I got it from Waterstones as I recall. It's not too big but does give you a pretty good guide. I'm not sure there would be a book purely about birds in Ghana.
Also try Amazon on the net.
German Amazon (Amazon.de) sells the US versions too, so I guess the UK branch should have them as well. Though .com may give a discount on the list price, but then, that might be offset by more postage.