https://www.kickstarter.com/projects/2032186437/secret-drugs-of-buddhism-a-book

It seems the author previously had an earlier version of the book available online but has since taken it down. It can be viewed here,

I've already posted a thread about the use of entheogens in Vajrayana. It has more references,
http://www.dharmaoverground.org/web/guest/discussion/-/message_boards/message/5733823

I don't have much commentary to add beyond saying the author seems sincere and admits the title is sensational (if you were put off by that too). I wonder to what extent the notion of 'amrita' was a metaphor that was interpreted variously.