This is more of a note then a definitive solution. I realized I was almost done every level in the standard game and the levels I had left didn't have any books available. Turns out, the challenges are standard levels, you actually need to go to the menu and select "Bonus" from the menu.
This brings up a list of challenges you can play. Each one will have a different reward for completion. Namely, the books are there. You need gold stars to unlock the challenges. To unlock Endurance Challenge 2 (the one tied to other achievements and last challenge in the list), you need to earn all 90 Gold Stars in the Standard Game.