One book my kids and I have enjoyed as a fun supplement to Middle School Math is The Book of Perfectly Perilous Math by Sean Connolly. I bought the book based on an online recommendation, and we’ve really enjoyed it. I mostly use it as a source of good extra credit problems for math tests. With Saxon only having 20 problems per test, the stakes are pretty high, so I normally add one “perilous problem” as a 5 point extra credit option. It’s all or nothing! The kids have loved the problems and they really are great for building thinking and problem solving skills. There is room to work the problems in the book, as well as a solutions section for each challenge. For under $12, it’s a good addition to your middle school library!