My son just finished "Revenge of the Desert Phantom" for school, and it was one of those books he actually picked up without prompting. It's a Hardy Boys mystery that sends Frank and Joe into the thick of African jungles, truly pitting them against some of the toughest odds they've faced. They're on a mission to stop some shady characters trying to exploit a rare resource, and it quickly turns into a desperate race against time with danger around every corner. Definitely not a quiet adventure.
He really tore through this one, which was nice to see. He said the intense jungle setting and all the clever traps the bad guys set kept him guessing through the whole thing. There's a lot of action sequences, with things like chasing through ruins and dodging ambushes, and the writing is pretty straightforward, so he never got bogged down trying to figure out what was happening. It's a chapter book, so no pictures beyond the cover, but he didn't seem to miss them one bit. At almost 30,000 words, it felt like a good, solid read for him without being overwhelming.
For those keeping track of AR points, this one is an AR Level 5.6 and worth 5.0 points. My fifth grader found it pretty accessible for his reading level, and I think it would be a good fit for most kids in the fourth to eighth grade range who enjoy a good mystery. It's a decent length at 29,883 words, so it gives them something substantial to get through for their reading logs without being a multi-day commitment. He definitely appreciated getting those 5 points for his goal.
I'd say this one is perfect for a kid who likes fast-paced adventure stories and isn't afraid of a little suspense and danger in an exotic locale. If your child enjoys classic mysteries or anything set in a vivid, unfamiliar place, they'll probably get a kick out of Frank and Joe trying to outsmart a villain in the middle of Africa. Skip it if they prefer fantasy or stories with a much slower build-up, but for a solid mystery-adventure, it's definitely a good choice.