This is simply one of the most provocative and moving novels I've ever read. Henry Miller said of Hamsun's *Mysteries that, reading it, he felt he was reading the story of his own life, at a level where empirical differences were irrelevant. *Axel affected me that way. It is one of those rare great novels that are about a soul, not its relational or social determinants (although these are well depicted); offhand, I can only compare it to *Mysteries and to Pessoa's *Book of Disquietude. Weeks after finishing it, I'm still haunted by its sensibility. Tolstoy is invoked often in the novel; imagine Tolstoy rewritten by an agnostic Kierkegaard.
The person on whom it was based was the author's great-uncle. He lived from 1858 to 1919 and was a close friend of Sibelius. The author learned about Axel only while reading about the latter; the name was never mentioned in his family. Axel left no diary; this novel is that diary, covering his entire life, interspersed with third-person sections each of which is a superb (and often heartrending) set-piece. If you don't know Sibelius's music, you will still, I think, be affected by it. But if you do - Axel was the person to whom S. dedicated his 2nd Symphony.