I'm working through that course right now. It's great.
Simo references this book by Jonathan Weber. (I was fortunate to take a GTM training that Jonathan led with Bounteous last year.) Unfortunately, the book is now four years out of date and I'm not sure how useful it would be given all of the changes to GTM in that span.
Read https://www.amazon.com/Practical-Google-Analytics-Manager-Developers/dp/148420266X
and reference this blog https://www.simoahava.com/
It would help to know some basic HTML, CSS and Javascript. I would put more emphasis on the CSS and Javascript portion than HTML if you want to learn more about conversion tracking.