This book is such an excellent example how simple things can be made.
You all know it - you hear a melody, like it, and then can't get rid of it. It stays in your ear ... and then you want to know: WHAT IS THIS??
Now, this book is able to give the answer. AND: You don't have to be an expert in musics - it's as simple as counting from one to three.
Are you able to decide whether the next tone is higher or lower than the previous? Well, that's it, you can use the book!

The first tone is noted as a dot (.), since we can't say "up" (U) or "down" (D) or "repeat" (R). Starting from there, each following tone is noted as U,D or R. This is done for the first 15 tones, and the resulting sequence mostly identifies each theme in each movement and piece.
So, e.g. Beethoven's 5th is
.RRDURRDURRDUURRD (at least I hope it is ...)

You write down the melody you can't get rid of in R-D-U notation, find the corresponding entry and know what it is.

I LOVE IT! I NEED IT! My parents (music teachers) depend on it.
And you will as well.

It is out of print, but if you can get one - I've seen auctions and offers for more than 350$!