If you can afford to spend a few bucks, hit up Amazon for a copy of Charles Roth's programmed text Use of the Dual-Trace Oscilloscope. It is ancient (1982), but still useful. Two, maybe three full generations of EE students have learned the basics of scope fu from this book (or its predecessor).
Full Disclosure: I own a copy, although I never used it in class, and instead learned my scope fu hands-on. I had Dr. Roth for two classes, Back In The Day. He is one of the best teachers I've ever encountered, and that's saying something, as UT Austin had a LOT of good ones. He's the only professor I ever encountered who could write a test that would take me exactly the time alloted, cover exactly the critically important stuff, not cover any of the fluff, not throw any trick questions, and still leave me limp as a wet dishrag at the end of it. I think that must have been his superpower.