The ABC of Piano Playing is a three-volume set of music books for children edited and compiled by Canadian piano teaching legend Boris Berlin. The books teach introductory piano technique and music theory using a middle C method (where students learn to play using both hands, with middle C being the "home" position for both thumbs).
The series is published in both English and French by the Frederick Harris Music Co.
Each book consists of a series of conncted lessons and pieces; the lesson might go over a new note duration, interval or technique, and the piece itself provides an opportunity to hear it in action.
With repertoire pieces ostensibly "about" subjects such as school, Halloween, sports and hopscotch, the books are plainly geared towards kids; the subject matter and tone is designed to keep them interested. (As an aside, the eight-year-old girl who was raised a die-hard Montreal Canadiens fan didn't see the connection between the soft, lyrical piece titled "We Play Hockey" and actual hockey, but I digress.)
Many of the books' pieces were composed by Berlin, though others are his arrangements of popular folk songs (such as "Old MacDonald" and "Sur le pont d'Avignon") or classical works by the likes of Paganini and Haydn. The initial pieces are as short as a few measures, building up to more complex pieces with more advanced techniques including chords.
Each volume concludes with arrangements of "O Canada" and "The Star-Spangled Banner."
The series is designed to prepare students for the Royal Conservatory of Music's first grade of piano study. From there, it's up to the teacher to decide what materials to use, though the Frederick Harris Co.'s Celebration Series was the most popular "next step" when I was a kid. (There's now a Celebration Series "Introductory" level book, which I don't think existed when I was starting out. My understanding is that it's sometimes used instead of The ABC of Piano Playing.)
Those books took me from a three-note song about bells (and you thought three chords was tiresome!) to a simplified arrangement of the Surprise Symphony.
Judging by the copyright date in my copy of book three, which I recently dug out of my parents' piano bench, the series was first published in 1985.
References:
The ABC of Piano Playing — Frederick Harris Music Co.
Piano Methods Explained