I meant to use diamonds to buy the 12 “building and survival” books I needed, but instead I ended..

I don't believe there is any way to reverse a diamond purchase like this in the game. But the good news is that earning more diamonds gets easier as you can get them from many rewards.

You can also check the Gift Codes list for lots of in game reward codes that will let you claim free diamonds if you haven;t already done so.

https://www.appgamer.com/puzzles-and-survival-gift-codes