I was lucky enough to pick this up at half-price, and would have been blown away by the value I got from my money, had I not been expecting the usual brilliance of Kevin Crawford's works.

For anyone planning an OSR game with domain management elements, strongholds, mercenaries or mass battles, this book is pretty much mandatory. Most OSR games and retro-clones tend to fall towards extremes when it comes to these aspects, providing too little detail, or far too much to be fun for most groups (looking at you, ACKS!). An Echo, Resounding hits the nail exactly on the head, letting your group have a meaningful domain management system and mass battle system without bogging the game down in, say, the finer points of tax collection and environmental adjustment to demand, or hiring armourers, tailors and horsefeed to keep your armies functional.

To give a brief (but by no means exhaustive) overview of the chapters:

1. Creating a domain: The domain creation system is quite simple but provides plenty of scope to make a unique and interesting setting, painting in broad strokes the major population centers, ruins, quirks and resources that characterise it. Random tables provide a wealth of options to instantly flesh out cities and wilderness areas.
2. Domain Management: This chapter outlines a domain turn system in which the characters' domain may take actions, followed by any NPC domains. Think Civ or any 4X game and you're not a hundred miles away. You can also create, destroy and upgrade locations on a smaller scale, example creating a city barracks to eliminate some of the upkeep for your standing troops.
3. Mass Combat: Mercenary/Soldier profiles (including many monstrous/non-human troops), an elegant combat system for battles of any magnitude, and rules for support troops, heroes, sieges, naval battles etc. You've probably seen something like it before but I say this honestly, this is easily the best implementation of any of these things I've seen in an OSR game (and I have seen MANY).
4. Westmark: An example domain that integrates all of the above, with plot hooks for good measure. I haven't looked through it in great detail but judging by the standard of the rest of the book, it's probably worth a read even if you don't plan on using it within your setting.

So in case you haven't already guessed, I'm very impressed with this product! I heartily recommend it to anyone who has even a passing interest in growing their campaign past fighting things and taking their stuff (or even just those who want to fight an army of things for a great deal of stuff).