Dear Aunt Debbie,
You have struck on the one genre of children's/YA books with which I have virtually no experience. I'm sure that my students read a ton of action/adventure books, but I haven't picked up on any in particular, and it's not an area that I found attractive as a reader myself.
The closest I can come is mysteries. My mom commented on your last post that your now-16 year old boy reader might like the mysteries of Josephine Tey. I remember devouring them in high school, though I haven't read them since. My parents have recently rediscovered them, and are complete converts.
Tey is perhaps most famous for The Daughter of Time, but the book I remember most strongly, and the one which I think might appeal most to reluctant readers, is Brat Farrar. It's a gripping mystery, with a few indelible characters and images, very much an exploration of deception. It's England, late-1940s. Brat Farrar is a 20-year-old British orphan who has become an expert horseman working on an American ranch, but knows little about his own background. Back in London, a man on the street mistakes Brat for Simon Latchett, the heir to a large estate in southern England. It turns out that Simon once had a twin brother, Patrick, who disappeared at age 13. Patrick was 15 minutes older, and would have been the heir...but then he disappeared. Brat is pressed into posing, somewhat reluctantly, as the missing Patrick to try to inherit the family fortune.
There are horse-related action scenes, murderous plots revealed, appealing and quirky family members, and the discomfort as Brat is romantically attracted to Eleanor, Simon and Patrick's slightly younger sister. Great drama! Totally satisfying climax! It's a really good book.
Come to think of it, there is a ton of action and intrigue in the other mysteries I read obsessively in junior high school: Agatha Christie. I must have read 50 of her books, easy. But I'll save her for another day.