There are few books I've read that I would call a unique experience to read. A few come to mind, like The Fountainhead, A Clockwork Orange, etc., but I can say without doubt that House of Leaves is by far the strangest experience I have ever had reading a book.

The book chronicles 2 stories happening, that of the Navidson's and that of Johnny Truant, who reads about the Navidsons alongside the reader, and provides extensive footnotes and stories about his own life. His footnotes throughout the book essentially chronical his descent into insanity and at times you can almost feel yourself slipping away with him. His sections are written brilliantly, and it really shows off Danielewski's skills writing in first person narrative. It's a delicate balance switching back and forth between first and third person, and he pulls it off.
One obvious thing to mention is the way the text is set up. In some sections of the book, there may be as few as one or two words or sentences, or the text may move in spirals or along the margins, adding to the suspense as the reader frantically flips the pages, left in the dark and frantically trying to figure out what's happening.
I will warn potential readers: though I loved it, it was by far the most disturbing and unsettling book I've ever read. This is the opposite of light reading. Be prepared to dedicate several days out of your life to read this, because you simply won't want to stop. Don't worry, it's worth it.