Ranger's Apprentice #8: The Kings of Clonmel by John Flanagan

My eleven year old is reading this series to himself.
Halt, Will and Horace investigate a neighboring kingdom, Clonmel.  Its leaders are promising defense against marauders in exchange for riches.  Something sinister is bubbling.
*Highly recommend.
-Borrowed from the library.
The library labels it "young adult".