The first two volumes of the Rhythm Book have been published by Hal Leonard. In order to purchase the first two volumes you can order them online or get them at an outlet that carries Hal Leonard’s books (the books are just recently published, so you may have to ask the outlets to order it for you if they do not yet have it in stock).
You can find a list of their online retailers here
… and a list of their brick-and-mortar outlets, organized by country
In many countries, you can also order them through Amazon. For example, in the USA, the Beginning book is available here
… and the intermediate book is available here
The other four volumes are being published by Rhythm & Dues. You can order them from Amazon.