Proofs: A Long-Form Mathematics Textbook (The Long-Form Math Textbook Series)
An Invitation to Combinatorics (Cambridge Mathematical Textbooks)
Math Practice Workbook Grades 6-8: 1000+ Questions You Need to Kill in Middle School by Brain Hunter Prep (Arithmetic, Algebra, Geometry, Measurement, ... more in Kill It Series by Brain Hunter Prep)
Book of Proof