David Hall 
There’s More to Playing the Piano [EPUB ebook] 
A thorough explanation of music theory with practical keyboard activities and video links for each topic

Understanding music theory enables you to learn repertoire faster and to interpret it more authentically. It also opens up the worlds of improvisation and composition. This e Book explains musical notation and theory in a concise, interactive style and covers everything from the very basics to a point just beyond Grade 5. Each chapter has a practical activity and a QR code or link to a video explaining topics covered further.