The Origins of the British, by Stephen Oppenheimer, is the only book I've read on the subject. It comes to the conclusions that lil_shepherd mentions.

I wish all this stuff had been around when I did the Anglo-Saxons at Oxford. I read loads of works by historians arguing madly about issues of continuity vs. radical change at the time of the Anglo-Saxon invasions, but none of this genetic evidence was available back then. (Or, at least, didn't appear in any of the books my tutor told me to read.)