"Challenges for Games Designers" by Brathwaite & Schreiber. This was by far the most helpful, as it laid out a process of finding & refining fun game ideas.
"Game Feel" by Swink. Examines why jumping feels better in Super Mario than Donkey Kong, why Street Fighter feels the way it does, etc. This way of breaking down & examining mechanics was eye opening and can be applied to other kinds of games.
I found "Art of Game Design" to be helpful later, when working on making a good game into a great game. The different lenses were great for finding areas where I could improve a game.
Finally, an observation & a suggestion, if you're open to them: You admit that you don't know game design, yet you're attempting to innovate by removing limits. This seems like an enormous challenge for a beginner, like someone who doesn't know how to play an instrument attempting to make a new style of jazz.
I think you'd have more success if you learned to play the instrument first: Practice by making a lot of small games. Spend 2-4 days on each, no more. Learn to use prototypes to explore the design space, searching for ideas with a glimmer of fun. When you find one, iterate on it to see if you can refine it into something more fun. As your game design skills improve, your ability to innovate will improve too.
"Challenges for Games Designers" by Brathwaite & Schreiber. This was by far the most helpful, as it laid out a process of finding & refining fun game ideas.
"Game Feel" by Swink. Examines why jumping feels better in Super Mario than Donkey Kong, why Street Fighter feels the way it does, etc. This way of breaking down & examining mechanics was eye opening and can be applied to other kinds of games.
lostgarden.com, especially https://lostgarden.com/2008/12/06/post-it-note-design-docs/, which describes a process for rapidly iterating on ideas, and https://lostgarden.com/2006/10/24/what-are-game-mechanics/, which introduced me to some of the fundamental building blocks of game mechanics.
Resources which helped me later:
I found "Art of Game Design" to be helpful later, when working on making a good game into a great game. The different lenses were great for finding areas where I could improve a game.
https://www.youtube.com/watch?v=Fy0aCDmgnxg really drove home how important polish is. This was useful after I had a game which was already a bit fun.
Finally, an observation & a suggestion, if you're open to them: You admit that you don't know game design, yet you're attempting to innovate by removing limits. This seems like an enormous challenge for a beginner, like someone who doesn't know how to play an instrument attempting to make a new style of jazz.
I think you'd have more success if you learned to play the instrument first: Practice by making a lot of small games. Spend 2-4 days on each, no more. Learn to use prototypes to explore the design space, searching for ideas with a glimmer of fun. When you find one, iterate on it to see if you can refine it into something more fun. As your game design skills improve, your ability to innovate will improve too.