Ask HN: Do you recall any book or course that made a topic finally click?

Sometimes it takes a book or a course (or explanation from a mentor) for a topic to finally click for you that you were struggling with for a long time.

For me, it was Stanford's EE261 course that made Fourier Transform click for me. Here is the link: https://see.stanford.edu/course/ee261

Similarly for deep learning it was fast.ai courses.

For programming it was How to Design Programs at www.htdp.org.

Your topic of choice may be anything, not necessarily CS.