All teachers were asked to have a small classroom library for students needing a book during SSR. I started going to Goodwill, Half Price Books, and local book sales to get books I had already read. I also paid attention to what students were reading and bought those. Students started giving me book recommendations, so I would get the book, read it, and then put it in my library (very important to screen books first!).
Here is what my classroom library looks like today:
But the part I think is most important is that I have read almost every single book in my library. Students come to me for book recommendations all the time. As their math teacher, I am not telling them what they should read. Rather, I ask them what they're interested in or what they've read recently and make a recommendation (or two). Last year, a former student was going on spring break and emailed me for a book recommendation--3 years after I had her in class! Students have even told me I have the best classroom library in 7th grade :) Their interest and happiness keep me going!
I keep a few Algebra 2 textbooks, math books (such as G is for Googol), and "Choose Your Own Adventure" on the bottom shelf. I love having a variety of books available to my students at all times!