How are you buying all of these books? They're so expensive

I'm seeing kindle books for 15 dollars, sometimes even 20 dollars. Any tips to get books cheaper? (not sideloading, I still want all the functionality with goodreads)

EDIT:
This post reached more people than I thought and I got some amazing recommendations. (I should've probably noted that I reside outside the US, where Libby is not really an option and where KU is not available)

I've read all the comments and these are the key takeaways:

  • Use the Libby App
  • Allocate a specific budget towards buying books for the year/month
  • Add books to amazon wishlist and check for discounts regularly (sort by price)
  • Bookbub, ereaderiq, freebooksy for tracking book deals
  • Stuff Your Kindle days (only for KU?)
  • Project Gutenberg

TYA