This has probably been posted before but since Category Theory has been popping up every once in a while I thought I'd share this. I'm just starting this and the instructors' [book](<a href="https://arxiv.org/abs/1803.05316" rel="nofollow">https://arxiv.org/abs/1803.05316</a>) here, and the subject is very approachable even for biologists like myself.