What makes a YA book a guy book?
Is it that the author is male?
That the protagonist is male?
That there are lots of references to Kevin Durant and LeBron James?
Or is there something more mysterious that appeals to guys?
YA Guy is going to go with the latter.
Authorship doesn't mean a thing. One of the best books I read as a preteen was Judy Blume's Then Again, Maybe I Won't. I didn't care that it had a female author.
Ditto with the narrator or main character. Then Again had a male MC. But I loved lots of Blume's books, including Deenie and Blubber, whose narrators were girls. I love The Fault in Our Stars, whose narrator is a teenage girl with terminal cancer. I love The Hunger Games, whose narrator is the poorly named but incredibly well realized teenage girl, Katniss Everdeen.
And okay, Katniss does hunt with bow and arrow and kick lots of butt, so you might think that's what makes the book appeal to my testosterone-driven existence.
But actually, what I like most about her is the choice she makes, the choice to save her sister. I think that's what most readers, male and female, find most appealing about her. It's not only the book's defining moment--the "inciting incident," the incident from which everything else follows--but one of the great moments in all literature. Who couldn't like a character who sacrifices herself to an uncertain fate in order to protect her little sibling?
So there you have it. Guy books are books that enable guys to see the best in themselves, to envision themselves as they could be even in the worst of worlds: as protectors, decision-makers, brave souls who stand up for others and for what's right.
That those are not exclusively "guy" characteristics is, of course, why such books appeal to male and female readers alike.
Let's write more books like that, and guys will read them.
Here's a list from J. King of YA books for boys. Note what's at #1.