Can you sell back your used books? I’ve had some moderate success with Amazon (enough to justify the small effort involved, sometimes even making a profit over the original cost of the book), especially if you plan to buy 2–4 books per month.
I think this post is setting up the idea that you have to keep books you buy. You want to buy books to support an industry that’s important to you. But you shouldn’t have to keep them all, especially when space is an issue and clutter bothers you. Think about what you would keep if you had to rent a storage unit — extra space in an apartment is similar.