By Louis Sachar
A boy is sent to a brutal desert camp for a crime he didn't commit, made to dig one hole a day, and slowly figures out the camp is digging for something.
Stanley Yelnats is wrongly convicted of stealing a pair of sneakers and shipped off to Camp Green Lake, which is not green and has no lake, just a dried-up flat of cracked dirt in the Texas heat. Every boy there has to dig one hole a day, five feet deep and five feet across, and the warden says it builds character. But nobody digs for character, and Stanley starts to notice the holes aren't random. Woven right through his story is a second one, a hundred years back, about a curse on Stanley's family, an outlaw called Kissin' Kate Barlow, and the real reason the lake dried up. The two stories dig toward each other until they meet.
This is the rung where the reading gets cleverer, not just longer. Sachar runs two stories at once, Stanley's now and a family curse a century back, and lets your child feel smart as the pieces click, because every odd detail turns out to matter by the last chapter. It's the perfect second step after a straight-through story like Wonder: same comfortable length, but now the book is asking your child to hold two threads in their head and trust that they'll braid together. Children who like solving things love it, and it's funny and fast even while it's building something intricate.