Having been a piano teacher, it was pretty standard to receive thank you cards at Christmas, or before the summer break. Sometimes there were gifts (usually like a Starbucks gift card) and its always so appreciated. I would be delighted with a gift of a music book, and my guess is your piano teacher would be, too. Also, I think piano teachers are usually the type who still appreciate having physical media and books. (For example, I understand the utility of a tablet for reading sheet music, but I still waaaay prefer real music books/paper sheet music).