That was my exact experience as well during development. I had the original EGA documentation from IBM as a really badly scanned PDF and the book "Programmers guide to the EGA, VGA and SVGA graphics cards", but that was it.
All of them were also not very good at describing anything related to best practices etc, but more or less only listed all the registers and memory addresses. So I had to do so much experimentation and learn stuff myself the hard way while I was developing.
Now that I build up a small library of all my EGA routines and know a lot more about how I could do things better and faster, i'm almost tempted to do another game based on all that.
I was also considering actually writing some tutorials/articles about it, so other people might be able to follow in my footsteps as well.
Sadly not many people have EGA monitors anymore. Even the hardcore retro computer nerds who have real CRT monitors don't have an EGA one, only VGA, which makes kinda stupid to limit yourself to an inferrior standard if you can just use mode 13 instead :) only if you are like me, and see it as both fun, challenging and a nostalgic factor.