For some reason I have locked into mysteries of the '40s and '50s. So I am reading things like the Mr and Mrs. North mysteries by the Lockridges and Nero Wolf by Stout.
I spent a large part of my childhood wanting to either be a Bob-White of the Glen or have secret clubhouse in a trailer buried in a junkyard. 
And a chauffeur-driven limosine!
You know… when it comes to books, I don’t have any guilty pleasures, just pleasures. Oh, I read plenty of lowbrow stuff, but I’ve never been embarrassed to be “caught” reading an old Mike Hammer paperback or an old pulp novel.
By contrast, I AM a little embarrassed when my wife finds me watching some awful movie on SY Fy, or singing along to some cheesy Seventies song on the radio.
As I said upthread, I consider Rex Stout to be great lit, thus no guilt for that! 
No shame in that. I recently re-read The Dark Is Rising, and then read the rest of the series. It was good writing when I was twelve, it’s good writing now.
My guilty pleasure is Janet Evanovich’s Stephanie Plum mysteries. At this point, I could probably write one myself. Just have to remember to include:
Stephanie gorging on Italian food from Giovannichi’s and fried chicken from the Clucky Bucket, yet not gaining an ounce;
Stephanie angsting over whether to sleep with the impossibly sexy, constantly horny yet devoted to her and his dog Morelli, or the even more impossibly sexy, constantly horny, dark and mysterious Ranger;
Lula shooting something inappropriate;
Grandma Mazur raising hell at a viewing;
Stephanie’s current car either being stolen, blown-up, set on fire or vandalized, forcing her to drive Uncle Sandor’s giant Buick; and
Implausible banter with a murderer, kidnapper, or recalcitrant skip.
Throw in a reference to Connie’s Mob connections, Lula’s weight and hair color, and Vinnie’s tryst with a duck, stick a numerical title on it, and it’s ready for the publisher.
You mean Archie didn’t pen them??
The PIN of the person in front of me at the ATM or the supermarket line.
Thing is, I don’t see YA books as guilty pleasures - a good story is a good story, even if the swearing and sex have been sanitised out, and that’s often the only real difference.
A real guilty pleasure, for me, is Piers Anthony’s Xanth series. Juvenile is the kindest word I can think of to describe it, but the first three or four I enjoy. After that, it gets too creepy even for me.
Funny books are always my favourite :U
Other than that I enjoy reading Thrillers, I’m currently reading Theodore Boone: The Activist, it’s great!
I also started reading Inferno by Dan Brown, my mom suggested it but I never finished it because I read it on a borrowed e-reader.
Once I’m finished with the Theodore Boone series I will start reading the books from Dan Brown, and start from the beginning.
I love “cozies” (especially dog-related) and “historical” fiction (Forever Amber, The Other Bolyen Girl, stuff like that).
Luckily it isn’t readily available to me, but if I found a stack of Silver Age comic books, I wouldn’t be able to stop until I read them all. It wouldn’t matter what the title, except maybe “romance” comics, or those terrible Bob Hope or Jerry Lewis comics. Yes, those were real things.