I enjoyed this enormously. I knew Kim Philby's 'official' story - who doesn't - but what really intrigued me here is that Mr Littell's version could be the truth just as easily as the 'official' version could. I liked the way in which the story was told from the viewpoints of different people who were acquainted with Philby personally and professionally across the years; all of the 'voices' were good, but I thought the chapters told by Guy Burgess and Theodore Maly were particularly well done. The descriptions of how one after another of the NKVD officers involved with Philby were caught up in the meat-grinder of Stalin's paranoia-fuelled purges conveyed the arbitrary terror of those times as well as any more scholarly historical work I've ever read.
Just one thing - as others have pointed out, there are a few occasions when Mr Littell's knowledge of British English fails him a little. Sometimes it's too modern for the time in which the scene is set, sometimes it just isn't British English, but American. It's particularly jarring when the words emerge from the mouth of an Englishman of the 1930s - 1950s. Somewhere along the line, some rather more meticulous editing might have solved the problem.
The final scene in SIS headquarters with Philby's 'sainted' father was brilliant; rather than tie the story up neatly, it left the question marks about Philby's real loyalty hanging in the air on the end of queries that will probably now never be answered. The 'official' line and Mr Littell's both seem, to me, to be equally plausible which, I suppose, neatly sums up the essence of loyalty and betrayal and the ambiguity inherent in spying - to a very great extent, it depends on which way you look at it.