A few years after getting that book, I started to work professionally with people building games, mostly white-labelled projects or contract work, which was the studio's bread and butter. But actually working with people developed my understanding of a) the relative value that artists, designers, and animators bring to the project, relative to my own set of skills and b) how to solve the sorts of problems that ship games. I am a programmer, and I use my programmer skills to give designers and artists what they ask for.
All of that to say: the best next step is working with folks, ideally some who have had experience. Book learning will only get you so far.
That being said, here are a few more books I have seen recommended in my sphere:
- The Timeless Way of Building by Christopher Alexander (this is not about games, but design generally)
- Finite and Infinite Games by James P. Carse
- Game Engine Architecture by Jason Gregory (this is a technical book about modern game engines)
Finally, Liz England (a designer who lowkey helped me not lose hope when I was breaking into the industry) has/had a blog where she talks about game design books. I cannot personally vouch for the titles, but I trust Liz England [1]