The Ranger's Apprentice #9 Halt's Peril by John Flanagan*

My eleven year old read this book to himself.
Will and Halt are pursuing a cult called the Outsiders.  The cult's assassins ambush Will and Halt, and Halt is near death from a poison arrow.  Will travels through Grimsdell Wood in search of the person with the antidote, Malkallam the Sorcerer. 
*Highly recommend.
-Borrowed from the library.