There are a whole lot of guides for sale on how to get into Oxbridge, but this isn't really one of them. It's not that it won't help you, but it's a whole lot more entertaining than that.
There are a lot of myths about the Oxbridge entrance procedures, a lot of them true (somehow I made it into Cambridge, so I know). But the greatest myth is that the questions they ask are somehow impossibly esoteric and unanswerable. In truth, the questions they ask relate to the subjects they teach, and test not just intellectual rigour, but intellectual curiosity.
This book will appeal to the intellectually curious. The rigour is there in the hands of the author, who has covered an impressive amount of ground in researching and contemplating the answers to interview questions from across the spectrum of subjects. (A quick Amazon search, and you'll see he's pretty well qualified to do so.) For every question, taken from real interview, you get a quick few pages' worth of insight.
This is fun, just for the drama of seeing the question, thinking, "God, how would I answer that", and then rolling on to a reassuringly clear and well thought out response, and then wondering, "could I ever have come up with that?". Always interesting, always entertaining, this is highly recommended. (And a good gift for anyone who might be applying.)