I saw the tv series first, wish I hadn't. Anyway, the book is a retelling of the activities of Kim Philby, from uni through to life in Russia. Absolutely fascinating. I'm most of the way through the novel, written by Ben Macintyre. The TV series is necessarily much abbreviated, I think casting that nice bloke from Band of Brothers was a bit of a mistake, he doesn't seem to fit the written description, but Guy Pearce was brilliant as Philby.
So if you want some good TV, watch it, but if you want the detail, hit the book.