Advice?

Hello everyone! So I've come across a book-related problem recently and I am seeking out advice.

To put it simply, I have run out of room on my bookshelf. My room is very small and I can't really see anywhere else I could store my books. I don't have the money to buy an extra bookshelf, either.

Where do you store your extra books? Have you ever experienced a bookshelf 'overflow'? What do you suggest I do?

Thank you!