I didn’t take any discrete math in college, so I’m trying to make up for it by slowly working through Kenneth P. Bogart’s Combinatorics Through Guided Discovery (available for free under the GNU Free Documentation License—the late Dr. Bogart was a mensch).
For now, I plan to use Org mode to show my work because it’s still relatively easy to use LaTeX within it, and it’s also relatively easy to convert to HTML.