This novel is adequate for a historical spy novel. That needs to be stated before anything else. Set in the early days of the Cold War, paperwork falls into the hands of an immigrant from the Communist Bloc, who tries to give it to the British but fails. The paperwork confirms the existence of a mysterious Third Force trying to play the Eastern and Western blocs against one another for sheer amusement. The inclusion of historical figure Kim Philby, a notorious traitor who was playing both the British and the Soviets himself, is a very skillful stroke.
As a Doctor Who novel, it seems forced. The Doctor seems to have been included simply perforce because Terrance Dicks is supposed to be a Doctor Who novelist. He has plenty of opportunity for globetrotting -- in the course of the novel, he makes it to France, the United States (where he meets President Truman), and Russia (where he rescues Joseph Stalin from evil influence).
The problem is, this could just as easily be any schlameel who happened to be in the wrong place at the wrong time and got blackmailed into involvement in the Cold War. Apart from his usual acumen, there's no reason the character should be the Doctor. His inclusion seems like a market-oriented contrivance that neither contributes to, nor detracts from, the novel.
The short version is, this is a good spy novel. The historical elements of the Cold War are very well done and the book is fun to read. However, it feels like it's only a Doctor Who novel because the author was under contract. It offers nothing new to the Doctor Who mythos. It detracts nothing either, so there's no reason it shouldn't be read; but there's no real reason for the character in it to be the Doctor.