The book is available from all major online sellers. Here are just a few of them in USA:
Amazon US
https://www.amazon.com/Reality-Decoder-Light-Adi-Andrei/dp/1036991830
Barnes&Noble
https://www.barnesandnoble.com/w/reality-decoder-adi-andrei/1150459388?ean=9781036991838
…and in the UK, EU, Australia, Japan, Rest of the World:
Waterstones
(not yet in this store)
Amazon UK
https://www.amazon.co.uk/Reality-Decoder-Light-Adi-Andrei/dp/1036991830
Booktopia (Australia)
https://www.booktopia.com.au/reality-decoder-adi-andrei/book/9781036991838.html
Bookfinder.com
https://www.bookfinder.com/isbn/9781036991838/
紀伊國屋書店ウェブストア ( Japan )
https://www.kinokuniya.co.jp/f/dsg-02-9781036991838