[edit: I am just now realizing how many ways there are to interpret this topic. I pretty much went with "what makes me check a book out of the library," but I could have taken it literally -- what kind of cover or title catches my eye enough to physically pick the book up and examine it further -- or what will convince me to actually buy a book.]
1. Being horse-centric/horse-inclusive. Actually, just about any mammal will make a book shoot to the top of my list. But the order of interest operations officially goes horse, dog, wild predator, cat, wild herbivore. Triple points if it's vintage, ideally published between 1930 and 1970.
2. Photographic covers / cover models. I grow weary of the YA industry's current fascination with
3. Hurt/comfort potential. This can be achieved through cancer or some other chronic illness (preferably w/ a happy ending), but I am also all on board with White Knights rescuing abused or just generally sad/lonely girls. Let me vicariously live my fantasies.
4. If it's a teen novel published in the 1940s-60s. They're just so solid and wholesome. (I actually just bought one at an antique store yesterday for $1.75, Fancy Free by Betty Cavanna. Mostly because Betty Cavanna, but I'm 10 pages in and I'm already awash in happiness)
5. Two words: Road Trip.
6. Anything involving big ol' haunted, abandoned, and/or historic houses/mansions.
7. Survivalist/post-apocalyptic scenarios related to a pandemic or natural disaster that disrupts society and technology. Zombies and aliens 100% excluded.
8. A small town or rural setting, particularly in the Midwest or West.
9. Unusual formatting -- regular text broken up with lists, emails, photos/drawings, etc., or just a graphic design book entirely, like Chopsticks or several of the other books on this shelf. Double points if it's 3-D, i.e. Griffin and Sabine-esque, with things to take out of envelopes.
(IS THERE A NAME FOR THAT KIND OF BOOK?? would really make it easier to Google for more options)
10. Books with associated playlists: each chapter relates to a song, specific songs or playlists are are referenced in / relevant to the plot, you have in-character videos on YouTube featuring original songs...
+11. Someone is reading an old diary and/or has found something in an attic that they use to solve a mystery from the past in the present day, typically featuring dual-timeline narratives.
+12. Nonfiction "books about books." The day I discovered there was an entire (small) section dediated to this topic in the library stacks was a good day.
+13. Nonfiction by a celebrity I love. Memoirs, cookbooks, style guides -- you name it, if someone I love is talking at me on the page about their own experiences / interests, I WANT IT.
P.S. I have been having SO MUCH FUN reading everyone's answers this week. It might be my favorite one since I started following the blog (over a year ago)?? I am just so curious about what makes bookish minds tick. And people are writing their answers so creatively, too.