[LinuXY]

For more inspiration I'd check out the Quake engine source. It's quite the masterpiece. There's also been some modernization over the years by LordHavoc of the DarkPlaces engine https://icculus.org/twilight/darkplaces/ - I'm constantly impressed how well the HD remaster on DarkPlaces looks on modern 4k hardware.

[bluedino]

The Wolf3D source is probably an even better start. Much simpler, the bad part is that it's 16-bit Borland C.

Andre Lamoth's DOS game programming books are a good read as well, he used 16-bit Microsoft C. Sprinklings of assembler in both.