Here's the last few lines of the book:
Teddy said, "I don't know, Chuck. You think they're onto us?"
"Nah." Chuck tilted his head back, squinting a bit in the sun, and he smiled at Teddy. "We're too smart for that."
"Yeah," Teddy said. "We are, aren't we?"
***
Personally, I thought the book had a perfect ending. I think it is evidence that Teddy regressed back into denial about being Andrew Laeddis, and his illness will never be cured. Movie was somewhat different.