Siri Hustvedt's What I Loved is a hard book to get a handle on. It starts off as one thing, then has evolved into something completely different by the novel's end. What it starts off as is a story about a growing friendship between two men, Leo Hertzberg and Bill Weschler. Leo is a professor who buys a painting by Bill, an artist and is so enthralled with it that he tracks Bill down. The two become friends and stay such through a number of changes in their lives, including Bill's decision to leave his wife for his model, the passionate intellectual Violet.

Then, midway through, the dynamic of the book changes. Without revealing too much, I can only say that What I Loved turns into a mystery involving Bill's troubled son, Mark. The second half the story is engrossing, there's no doubt of that. But it signals a sharp turn away from where the book seemed to be headed at the outset. While I am all for surprises and interesting twists, the book seemed to lose its way once it strays for good from the dynamic between Bill and Leo.

Still, there is much to recommend it. Leo, Hustvedt's narrator, has a strong voice and, perhaps his best quality, seems just as baffled by where his story is heading as the reader. As an intellectual, he constantly tries to rationalize the world around him, but finds that the complexity of human behavior renders this impossible. As a protagonist, he is always convincing and usually sympathetic. I also liked the character of Violet who, like Leo, is an intellectual who nonetheless finds herself a slave to her emotions.

Though the book doesn't quite recover from its turn into the bizarre, it is challenging, surprising and intelligent, which definitely makes it worth a read.

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• The Sorrows of an American
• A Plea for Eros