I can't resist a historical romance. They're easy to read, you learn something (get your mind out of the gutter!) and they're highly entertaining.
I first read the Outlander series when I was in high school. I've taken to re-reading them whenever I can't find something decent to read. Claire and Jaime, the protagonists, are engaging and likable. I always want to see what sort of adventures they're having. There's time travel, war, life in the French court under Louis, and immigration to the colonies. I rarely re-read anything but these novels charm me again and again.
Shadowy Horses, the latest from Susanna Kearsley, is another engaging novel. Set in the Scottish borderlands, it's about the discovery of the resting place of the legendary Ninth Roman Legion. It's a mix of archaeology, history, romance, suspense and a little super natural. It's immediately absorbing.