Mogworld by Yahtzee Croshaw follows a sentient NPC unknowingly living in a video game. It's not so much wrapped up in nostalgia and is a dark comedy, but the author takes the premise seriously enough. I've read it several times. Not many books like it.

Ring World by Larry Niven is a big departure from the genre of video game existence, except that it's the inspiration for the Halo series. It's a old fashioned sci-fi romp, the kind of thing that inspired ready player one. It's got super technical cool hard sci fi concepts mixed with an extremely colorful cast of misfit protagonists. Very readable.

Random, but Mrs. Frisby and the Rats of NIMH by Robert C. O'Brien is so extremely imaginative in the way it nest it's cast of talking animals within the real human world. (Yes it is actually sci-fi). The book is marketed towards children, but that was more an artifact of how it got published back in the day. It's extremely well written. The movie is cute but takes a lot of liberties.