The book is available from all major online sellers. Here are just a few of them in US:
Amazon US
https://www.amazon.com/Reality-Decoder-Souls-Earthly-Experience/dp/1036912914
Barnes&Noble
https://www.barnesandnoble.com/w/reality-decoder-hitomi-sakamoto/1147530461
Bookshop.org
https://bookshop.org/p/books/reality-decoder-a-soul-s-guide-to-the-earthly-experience-adi-andrei/22757753
…and in the UK / EU / Australia / Rest of the World:
Waterstones
https://www.waterstones.com/book/reality-decoder/hitomi-sakamoto/adi-andrei/9781036912918
Amazon UK
https://www.amazon.co.uk/REALITY-DECODER-SOULS-EARTHLY-EXPERIENCE/dp/1036912914
Booktopia
https://www.booktopia.com.au/reality-decoder-hitomi-sakamoto/book/9781036912918.html
Bookfinder.com
https://www.bookfinder.com/isbn/9781036912918/