I think you mean about Fourier analysis, yes? Brad Osgood's Stanford lectures (available on YouTube) give a very good (read: steady, unassuming) approach and the lecture notes associated with that course are extremely detailed and informative if you want a deeper dive.

Yep, definitely Fourier analysis, I haven't studied it formally at all.

Thanks for the pointer to Osgood's lectures. A co-worker recommended Stein's "Fourier Analysis" after a bit of work on my part. Do you have any thoughts on that text?