The Hacker News thread discusses "Discrete Mathematics: An Open Introduction," a free interactive online textbook. Users share resources, including links to the book itself, its GitHub repository, related textbooks from the American Institute of Mathematics, and video lectures. Some commenters debate the book's suitability for high schoolers, with opinions diverging on whether the material is accessible to that age group. Some think parts are suitable and other's who teach math believe the concepts are too advanced. Some users share their personal experiences with learning discrete math and recommend other introductory texts. Some users engage in a debate whether calculus or discrete math is easier to learn and when the best time to learn the subject is.