These subjects tend toward course curriculums that are far less standardized compared to others. For discrete math, Discrete Mathematics: An Open Introduction by Oscar Levin is a particularly effective textbook produced with PreTeXt. On the combinatorics side, Combinatorics through Guided Discovery by Kenneth P. Bogart is another excellent PreTeXt book. Applied Combinatorics, by Mitchel T. Keller and William T. Trotter, and Applied Discrete Structures, by Alan Doerr and Kenneth Levasseur, are also deserving of consideration.