Young Adult Literature for...? Help!

So I've definitely been going for the "soft launch" approach on The Book of Beings. I've put the series out there, and I've been gradually doing the things that might get it into places where readers who will enjoy it are more likely to see it. But I haven't had a big marketing plan, and I haven't been following some crack promotional strategy.

Partly what that means is that I haven't thought too much about what my niche is. I know, I know. The marketing people tell you you absolutely have to do that. Somehow I just don't have the energy.  

But what I have noticed, now that the book has been out there for a while, is that a certain kind of reader really, really likes it. These readers tend to be over 30, sometimes significantly so. (One reader told me her 83-year-old mother couldn't put it down.) They also tend to typically not read much young adult literature; they're more literary fiction types.  

Now that I know this, I want to be  help these kinds of readers understand that this book may be for them, even if, at first glance, it doesn't seem like the kind of book they usually read. I feel like there ought to be some wonderful, clever, tagline--something tweetable--that will communicate this probable affinity to them, but I have to admit, I'm at a loss as to what it might be.  

I've poked around the internet some, and the two phrases that seem to come up most often to refer to these kinds of books are "YA books for adults" and "YA books for grownups." I have to say, both of these descriptions leave me a little cold. Let's just set aside the issue that one of the traditional definitions of YA is that it is written for 16-25 year-olds. I mean, aren't most of those people adults (albeit young adults) by pretty much any definition? Do we want to insult them by implying that they are not?

Besides, neither "adult" nor "grownup" has a connotation that makes it sound particularly appealing in this culture. Ditto "mature" and "older." I mean, it's not quite the same as saying "YA books for the staid," or "YA books for people with crow's feet," but still...

I have a similar problem with "YA books for sophisticated readers" or "smart people" or "intelligent readers," and so forth. Who wants to imply that some readers might not be those things and that those readers should just pass you right by? I'm not a snob. I love readers, all readers, and I can't see making friends by alienating people. That's just not my style.

There's a site about YA books called "Forever Young Adult," which is kind of cute, but obviously I'm not going to steal it. (And besides, whenever I see the name, I can't help hearing Rod Stewart singing "Forever Young" in my mind, and it's not a mental loop I want to cultivate.) Their tagline is   "A site for YA readers who are a little less Y and a bit more A." I'm pretty jealous of that. It seems to skirt the are-you-an-adult/are-you-not-an-adult thing nicely, not to mention that it cleverly avoids the use of the actual word "adult."

Buzzfeed put together a collection of "YA Books for Adults who Don't Read YA."  There may be some potential there. I could go with something like "A YA series for readers who don't read YA." Still. It doesn't have much zing...

Any ideas, readers? You got any extra clever taglines lying around you want to share?