I Capture the Castle, the first novel by Dodie Smith (of One Hundred and One Dalmatians fame) is utterly charming. Published in 1948 during Smith's stay in America, the novel is purportedly published directly from the pages of nineteen-year-old aspiring author Cassandra Mortmain's journal. While this sometimes causes the reader agonies of suspense ("A new chapter happened yesterday which I long to dash straight into, but I shall resist the temptation and bring myself up to date first."), the journal serves as an excellent window into the peculiar lives of the Mortmain family, including writer's-block plagued Father, social-climbing Rose, eccentric stepmother Topaz, sensible schoolboy Thomas, and pets Heloise and Abelard. Stuffed full of literary references, romance, well-developed characters, and the occasional philosophical musing, I Capture the Castle comes to a satisfying, but not cloying, conclusion.
ETA: I have the movie, and will post a review of it as well if I manage to watch it. I am trying not to go into it with a great deal of prejudice.
Further ETA: If you are choosing: read the book, the movie was a bit flat.