What pedagogy book is recommended?

What age of kids? 
 

For young ones, Music For Little Mozarts method books series has a comprehensive teaching guide.

Piano Adventures is extremely self-explanatory, and has a wide age range. 
 

For books about teaching piano, I’m not sure exactly. It’s always good to get some grounding in the Big 3 Music Educators: Orff, Kódaly, and Dalcroze. 

If they are younger than 10, Hoffman Academy is a very good resource. 

I really like Piano Bop books 1, 2 and 3 together with lesson videos by Jacob Koller. My guess is not many people have come across this before. 

It is not a pedagogy book, but the videos serve as a good reference to how to conduct a lesson that is suitable for little kids. 

The main strength of the books are that it is extremely fun. Every concept is introduced as a catchy song where the kid sings along and the teacher accompanies.

Each lesson has 3 videos, a lesson video, a play along video for self practice, and a teacher's play along example video.

The lessons introduces many concepts typically absent from classical type books. Improvisation is introduced right from the beginning. The student is also exposed to different genres of music such as jazz, bossa nova etc. There are lessons on playing from lead sheets, chord progressions, transposing songs etc. 

The teacher's accompaniment is professionally arranged with chord symbols written to encourage improvisation from the teacher as they play along. Personally, I found this quite fun as I get to see how simple kids tunes get turned into professionally arranged tunes with good chord progressions and embellishments. 

For advanced learners, I really like Fundamentals of piano practice, 3rd Edition by Chuan C Chang. 

This one is a pedagogy book.