TL;DR - A MASTERPIECE
Adaptable NPCs is what I like to call a "Texture Supplement". Have you ever wondered how a particularly good DM manages to make the world feel so lived-in? They've spent hours and hours on building up a massive backlog of NPCs and buildings and hooks, only to be pulled out when they need that added "Texture" to keep immersion.
The magic of this comes from the versatility it gives your games, it does ALL THAT HEAVY LIFTING FOR YOU.
Each new NPC is painted with new depth and ability, and adds so much to the player's narrative experience beyond "This is a Bandit" "This is a Bandit Captain". It saves you HOURS of having to come up with the content on your own, meaning you can focus more on the adventure.
There are few supplements that could add as much quality-of-life to a Dungeon Master than this incredible book. I am quite honestly sorely tempted to recommend this over the OFFICIAL DUNGEON MASTERS GUIDE for most DMs.