Xash3D FWGS (nonfiction)

Xash3D FWGS is an absolute marvel of community reverse-engineering. At its core, it is an independent, custom-written 3D engine built from scratch to be 90%+ drop-in compatible with Valve's GoldSrc formats (maps, models, entities, and audio), while completely shattering the hardcoded engine limits that plagued classic Half-Life 1 modding.