I've been getting source material that are open source or public domain like De Re Militari and using them as inspiration to my own studies and gaming material. They are Inspired by my operations research studies which has a very gamer mindset perspective in looking at things with stats.
If you want to reinforce that bias I recommend the works of Amon Tversky and Danniel Khanneman's works on decision making and judgement (use them as a search term or get thinking fast and slow).
These books which were tried and true for centuries, like De Re Militari, finds new life when i can model them in gaming stats grounded by some models I've developed for games and work. I've taken some heuristics and modeled them into game mechanics for open collaboration and improvement.
Imagine if you can make sense of De re militari in character templates and stats. What are the baselines and what are they when you look at real people vs the statistics? So detailed that you can be inspired to undergo that training as a hobby. That you know what kind of training is needed to do a 32km March 2x a month in 40-60lbs of gear (even if it's in the safety of a gym treadmill at an incline to simulate hard terrain).
Knowing what ( in an operations science way) a task, activity, and ability is and how to measure and schedule a plan to reach a performance level is another coloring of my gaming and my gaming mindset regarding my work studies. And the game is the exercise I want to do.
Primary sources colored by updated material. Imagine some gaming mechanics and entries drawing referencing de re militari, Wikipedia and, US Marine corps small wars manual. Sources that can help inform the reader the assumptions and hopefully someone can come up with a better model.
Public domain books Gdoc