I agree with Jim about the Peter Lapsley book. PL has something useful to say about all the UK stillwater disciplines - small waters, reservoirs, lakes lochs and loughs, bank, boat, imitative fishing, stalking. As Jim says it's a maybe a tiny bit dated but don't worry about that at this stage, if you want to update your tactics having caught a few fish on a few different waters, the two astonishingly useful Henry Lowe books are recommended;
http://www.anglebooks.com/product.ph...ngler-s-guide-
http://www.anglebooks.com/product.ph...isher-s-guide-
I am sure the Peter Cockwill book will be first class, but it may major on small stillwaters only.
Brian Clarke's book as JJO says is a good "back to basics..." on imitative stillwater fishing, but I would fear you might find it only tells you half the story, however well written the book is. Lures, fry imitations, and wet flies both traditional and more modern all have their place alongside nymphs and dry flies, and if I have learned one lesson in 37 years stillwater fishing (I did start very young

) it's exactly that - you should aim to be a capable allrounder even though you might slightly prefer one technique to others.