I've stumbled onto Envy earlier and I agree with you, there's not much out there about it except people talking about how much they love it[0].

I found this book[1] on Amazon in case you or anyone else is interested.

[0]: https://paulhammant.com/2017/09/01/smalltalk-envy/

[1]: https://www.amazon.com/gp/product/0521666503