A great book, finely edited.
Hugh Trevor-Roper as a young man was starting his academic career as an historian when World War II disrupted all life. This extremely smart and extraordinarily well read scholar served in British intelligence and, at war's conclusion, established the facts surrounding Hitler's death in Berlin.
Many of these journal entries have nothing directly to do with the war, but are random reflections on life, history, religion, fox-hunting, Oxford, and friendships with such people as Logan Pearsall Smith.
The introductions to many books are a waste, but one should read Richard Davenport-Hines' foreword, which sets the context for the materiel that follows. This careful editor also provides many piquant and informative footnotes throughout the book.
A good comprehensive biography is out this publishing season on Trevor-Roper :"An Honourable Englishman" by Adam Sisman. However, if you are forced to choose between these two books, I recommend that you spend your time with "The Wartime Journals."