Karen Collins book is considered the "standard" work about video game music at the very least in musicology. Plenty of research papers have been published in the recent years, go to your uni/college and search via their databases.
Also, don't want to be an ass here, but your topic is really, really big. Collins basically does exactly what you set out to do but her book is quite long. If you can still change your focus I suggest you find something a bit "slimmer" so you can maybe come with new theories and do some analysis/research of your own.
I'm writing my bachelor's thesis in musicology on game music. My thesis is a part analytical and part ethno-musicological study of Game Boy music, mainly the chiptune scene with focus on Sweden. 40 pages of examining all the modern music-making software on the gameboy, interviews with artists and fans about the scene, interview with LSDJ's creator and more. Gonna see what I can conclude about all this in December
Anyway, hope you find what you're looking for baby <3