…Turtles All the Way Down is somehow far darker [than The Fault in Our Stars], not so much because of the subject matter—though that's dark too—but because of how [Green] chooses to write about it. This novel is by far his most difficult to read. It's also his most astonishing…We spend long stretches inside Aza's head, listening to [her] swift and unsteady thoughts…her mind…in the throes of a civil war…If Green were writing in his usual register, he'd interrupt Aza's descents into these cognitive spirals…with a bit of humor. But he seems to have made a decision: If Aza can't find relief, neither can we…If an author has integrity, [his novel] should end plausibly. Green has integrity. He also has O.C.D. He's tweeted about it; he's discussed it on his famous video blog…I still wasn't prepared for the ending of this novel. It's so surprising and moving and true that I became completely unstrung, incapable of reading it to my husband without breaking down. One needn't be suffering like Aza to identify with it. One need only be human. Everyone, at some point, knows what it's like when the mind develops a mind of its own.
The New York Times – Jennifer Senior