I took a sabbatic from reading kids' books while I was on vacation. And, yet, three of the books I finished have a kid connection of one sort or another.
For instance, On the Wrong Track by Steve Hockensmith has what I would call a thematic connection to YA. On the Wrong Track is the second in the Holmes on the Range series, Western mysteries set in the 1890s. Our narrator is twenty-year-old Otto "Big Red" Amlingmeyer, a cowboy who wanders with his twenty-seven-year-old brother Gustav "Old Red."
Seems pretty remote from twenty-first century YA readers, doesn't it? Well, the thing is, Gustav wants to be more than an illiterate cowpoke of few words. He wants to be a deducifier like his hero, Sherlock Holmes. And his little (though physically quite enormous) brother, Otto, wants to write up their adventures and publish them like Dr. Watson did Holmes'.
Though these two red-headed brothers are twenty-somethings, they seem younger (to the extent that people who are handy with guns and foul language can seem young) because they're trying to determine who and what they're going to be. In this book, they run into a burned out, dime novel hero who is not what he once was and maybe never was. They have to deal, each in his own way, with a young, very intertriguing, woman. They are confronted with disappointment and all kinds of road blocks in pursuing their goals.
I'm not saying that On the Wrong Track is a YA book, but it deals with issues that are common in YA novels and that should have appeal for YA readers.
I also think that On the Wrong Track is a good historical novel. Many historical novels for younger readers are what I'd describe as unbalanced. A lot of attention has been given to the historical setting but characters are often underdeveloped or cliched and plots are weak. My own guess is that children's and YA historical fiction is viewed as being educational. Such books are supposed to teach something about the period and are given a pass on other elements.
The Holmes on the Range books, however, provide a strong setting, terrific characters who are at home in that setting, and real plots. Okay, a lot of those terrific characters use realisitic, coarse language, so you might not want to be the adult who hands off one of these things to a delicate twelve year old. But mid-teens will have heard it all before, and a good historical mystery could open their minds to the opportunities historical novels offer.