Read Huckleberry Finn if you haven't. It is a more grown up book than Tom Sawyer. Probably one of the five most important American novels.

Connecticut Yankee in King Arthur's Court is surprisingly bitter, but also a good read.

Also plenty of short stories are worth reading if you don't have time for a novel. A short amusing one about home security: https://americanliterature.com/author/mark-twain/short-story...