A few years ago, a friend introduced me to Butterick's Practical Typography, and I found it extremely helpful in improving my eye for and skills in typography. The guide is now in its second edition, and the author Matthew Butterick has only improved it. Typography in ten minutes is well worth the ten minutes.
I'm not entirely in agreement with everything Butterick proposes, but I think nearly every comment is rooted in a good insight. For example, I think its okay for line length to be more than 2-3 alphabets, and that closer to 3 or 4 is better than closer to 2. This personal preference also manifests in the code I write, as I prefer lines closer to 110 characters and not 80. Regardless, controlling line length for the purpose of better readability is definitely something to consider no matter the type of document.
I highly recommend reading the guide and supporting Butterick if you find the guide useful.