Daniel Marcus’s book, published by the Mathematical Association of America (MAA), is the gold standard for this style. It is designed specifically for students to work through independently or in a discovery-based classroom.
If you can't find the Marcus PDF or want to supplement your learning, check out these highly-rated "problem-first" books: graph theory a problem oriented approach pdf best
It trains you to think like a discrete mathematician, focusing on connectivity, planarity, and colorings through trial and error. Key Highlights of Daniel A. Marcus's Text Daniel Marcus’s book