TBH it looks pretty good. Covers lots of theory, lots of repertoire graded for ability, and I'm super happy to see it covers rhythm. I think a few people knocked it for typography, I'm not that picky. Perhaps one of the biggest problems with any book is what is the % of people that finish any book much less 9 of them? Realistically this seems better suited as material for a teacher to incorporate into lessons.
I think the one thing that apps and video lessons offer autodidacts that books don't is repetitive guided practice. Going through every scale & chord with you, guiding you though syncopation exercises, walking you through a harmonic analysis, etc...
From my perspective as someone who learned for about 8 or 9 years, there's a lot of theory here. Personally I never learned much theory - as a child, playing piano wasn't something I particularly enjoyed (and it's only now as an adult that I do it occasionally for fun and appreciate the time I put in), but back then learning theory was even lower on the list.
I do think theory is useful for understanding the mechanics of music, and may help accelerate learning any instrument.
My teacher pulled a lot of material from the Royal Conservatory of Music (and I didn't learn theory from them). I haven't heard of this course until today.
