TruthQuest Spine Books are always discounted at Schoolhouse Publishing!
TruthQuest author Michelle Miller recommends certain core pieces of literature that span the entire scope of the era being studied. She refers to these as "spines." I recommend that you keep 2 or 3 spine books available while you are working through each TruthQuest guide to help your student tie all his reading together.
Though they are recommended by TruthQuest, these books obviously aren't just supplemental to the TQ curriculum! In fact, the reason they are recommended by TQ is because they are such great books! Don't hesitate to add a few of these titles to supplement any history program you are currently using. Though not required in TQ, these books are referred to repeatedly and are called "spine books," because they're broad in scope, covering an entire time period. Each TQ guide recommends several spine books to give your student the full scope of that era. You don't need all the spine books (I usually have only 2 or 3 for any given era).