A Rulebook for Arguments, by Anthony Weston
This little book is recommended for 9th-graders in The Well-Trained Mind. It's a useful collection of 45 specific rules for writing or arguing correctly. It's clear and interesting, with good examples of common mistakes or good strategies. I wish I'd read it when I was in high school!
Five-Minute Marriage, by Joan Aiken
Joan Aiken is on my Favorite Authors list, and she was one of the few writers who could produce a really good Regency novel (Georgette Heyer is the other one that I know of). This one features Delphie Carteret, an impoverished and independent young woman with a slightly dotty mother to support who is forced to apply to her mother's estranged family for help. She is promptly dropped into an adventure featuring villainous conspiracies, imposters, a counterfeit marraige that may not be so counterfeit, and a whole pack of aggravating relations.
Aiken wrote several Regency novels, including some Jane Austen spinoffs that are actually worth reading. She is most famous for her "Wolves Chronicles" series for children, which started with The Wolves of Willoughby Chase and continues for quite a few wonderful volumes. And I can't miss mentioning the Mortimer books for children, featuring the young Arabel and her hideous, obnoxious raven Mortimer. We love those and if you have never read any Joan Aiken, I'm telling you that you should!