Agree, freely available extensive documentation is a must. The link to Book of Shen is for UK. To find it elsewhere, search for the isbn, eg. https://www.amazon.com/s?k=1915012112
Edit: The book is available online, but is scanned in low res blurry format, very not readable.
If you want to jump straight into what makes Shen interesting, read Part 10 of the manual, starting with recursive types: https://shenlanguage.org/OSM/Recursive.html