My own screwy words could hardly do it justice, and neither does Wikipedia's page (seeing as how it's an encyclopedia and not a literary critic), but here's an excerpt from its TVTropes page, paraphrased from a passage in the book:
Wait, no, that's still not right. Okay, you're interested in what House of Leaves is about, right? Well, this book is about that point directly behind your head. Don't look.◊Ω Don't take your eyes off this page, off the safe glow of the monitor, the comforting shapes of the letters making up this sentence. This is safe. What's behind you isn't. Keep reading these words. If you stop to look behind you, I can't guarantee you'll come out of this ordeal alive, much less sane. Pretty soon you might find yourself doubting what is real and what isn't. Pretty soon you might start to havethe nightmares. One day you'll wake up to find yourself an emaciated wreck who can't trust space and time anymore. Whether something is real or not doesn't matter here; the consequences are the same. What you need to realize is that this is not for you.
You can find some introductory readings online, I'm certain, and if you're interested in a complex, experimental, interlayered book that's sort of but not really about a Necronomicon-styled book written about a Blair-Witch-esque movie produced about a house with certain unorthodox geometries populated by a somewhat unhappy family, give it a google search. It's seriously the best book I've read in a long time. Google it. Look at the TV Tropes page and the Wikipedia page. It's brilliant.