A mystery that teens can read and stay interested
Thanks for asking! Let me encourage you to borrow the brains of better folks than me and fill out this questionnaire for a Teen Custom Reading List. The staff that monitor that system are some of our best and brightest youth readers' advisors. But I'll do what I can in the meantime. I can refine my recommendations, too, if you're willing to email back with further guidance on what does and doesn't seem like a good fit from the following. Since I've always been a fan of audiobooks for road trips, I'll link to a few books on CD where we have them available.
If historical mysteries are your cup of tea, there are a few solid options from our Young Adult collection where a younger female protagonist might help keep your granddaughter engaged. Consider Ying Lee's A Spy in the House, Anne Blankman's Prisoner of Night and Fog, or Julie Berry's Scandalous Sisterhood of Prickwillow Place.
If you'd like to tend toward books aimed toward a younger audience yet that may still have interest for both of you, consider Wendelin Van Draanen's Sammy Keyes books or Mary Downing Hahn's paranormal mystery Closed for the Season.
Back in the books we would classify as Young Adult, there are quite a few authors with plots that borrow the structure, if not the setting and characters, of noir-ish mystery fiction. Both John Green's Paper Towns and Jay Asher's Thirteen Reasons Why are good examples of that flavor. I will warn that though both were very popular books for teens there will be some language and intense themes including bullying, substance abuse, and the aftermath of sexual abuse.
We can try out a few different focus areas and types of mysteries if nothing here fits the bill. And of course, every reader is different and every family has a different take on age-appropriate content. But it would *absolutely* be worth your time to try out the custom reading list service linked above.
Thanks for using AskOPL!