Having played a LOT of RPGs since AD&D (ages and aeons ago) and bought far too many sources books, supplements, etc, FlexAI Guidebook is the single best purchase I have ever made.
These days, my group likes to play GM-less games (aka collaborative GM games) and Mythic GM Emulator holds the spotlight, howeveer FlexAI Guidebook is one of those crucial pieces that makes combats and encounters far more meaningful. There is no arbirtray GM decision. For my group this is vital, beacuse we don't want to bias NPC behavior in any direction.
This tome is over 400 pages and provides reaction tables for esentially every imaginable NPC type in almost every imaginable situation and scalable for different levels of difficulty.
The book breaks down into major chapters of NPC type: Solider, Skirmisher, Controller, Brute, etc. It's easy to assign an NPC to one of these categories. Within these categories it breaks down again to minion, normal, elite, etc. THEN it breaks down into current situation such as: unprepared, ambushing, bloodied, etc.
For example, in my own Lord of the Rings game, we have orc gruns (Soldier, Normal) and goblin grunts (Skirmisher, Normal)j and both will behave differently. As will ranged fighters (e.g. archers) and leaders (e.g. captains).
As good as this book is (every GMs friend and if you enjoy GM-less or solo playing I cannot recommend it enough) there is one issue. If you really want to go all-in on the detail, it will be vital to set up a chart to track each NPCs current condition. I use dice to match a specific NPC to a specific condition which provides a specific chart. Due to this, I recommend getting the PDF so that charts can be printed out to have onhand rather than constant flipping through the book. By the way, the hardcover is an amazing price to the content!
Because of the extra bookkeeping there can be a lot of overhead. In my opinion, this overhead is worthwhile because combats (and other events) become richer and more realistic. (My RPG of choice is GURPS which already can be slow in combat so this isn't such a big issue.)
This is truly an amazing purchase! At a minimum check out the sample and give it a try!