How do you click on the McM book shelf on Astro Knights Island?

You are trying to jump ahead of the game. You need the Library Slip from the Mordred Museum, which shows that Mordred was getting books from that section.

Get the gold coin from the fountain, pay your way into the museum, and find the Library Slip in the stack of books. Now when you go to the castle library, you can click on the non-fiction (lower aisle) where it says McM.