Reign Enchiridion is just the rules of the larger book Reign. Just rules, no fluff. Greg Stolze is one of the view RPG writers I can sit down and read for an extended period of time. He tends to be to the point and explains things when the need it.
Reign Enchiridion use the ORE (One Roll Engine). You roll a number of 10 sided dice and try to get matching numbers. A very poor roll would be something like two 1's. A very good roll would be something like five 10's. The results tell you pretty much the most important things you need to know. An attack roll, if successful, would tell you where you landed your blow, how much damage you inflicted, and how fast you did it. There is a bit more to the system than that, but in play it works out very well and actually pretty fast.
Reign Enchiridion offers you a lot of various options and spells and ways of creating characters and actually includes some of the supplements that Greg Stolze has written for Reign.
One of the main features of Reign is the Company system which allows you to create, build up, destroy and create conflict between various large factions like guilds, armies, countries. It is very very cool.
Somethings that might be problems for the game would be armor. A lot of the time you might be only get pairs and triplets and the occasional quadruple. Armor is very simple in that it's rating reduces the damage. Some of the armor is rated at 3. I think that's the highest it goes, but with my experience most damage would be mitigated completely even for less powerful characters. There are several quirks for ORE, but from what I hear it is mostly intentional. The system lends itself very well to deadly combat and of course if the armor thing is an issue it can easily be house ruled.
All in all Reign Enchiridion is very well written and also a very fun game. The company system can easily be used in any type of game not just medieval fantasy. In fact I found myself thinking that for several of the systems in the game. Very fun, I recommend it to any RPG fan and ORE fan.
Arc Dream's Paperback "digest" style mini game books are hands down one of my favorite gaming products. For the price of a single extra RPG handbook (which contains all sorts of stuff I don't want the players to see anyways) I can buy three or four of these and pass them around the table! It helps players to familiarize themselves with the rules, make characters, and get a sense for how the whole system works without monopolizing the $50 core book.
They also make a great introduction to a system if you're unsure about it. Ever wanted to know if ORE is for you? Pick this up! It's got everything you need to run a simple game.
Reign itself is one of my favorite fantasy systems, though I tend to use the setting info as only a loose guide (that's how I usually do things anyways, so no sleight to the setting there). It contains a great system for building up martial characters into interesting and unique combatants, with two examples in the mini book to get you started. The magic is a little "low magic" for my taste, but there are definitely stories where it works well and the system is solid and fun.
I was actually familiar with REIGN before buying this, through a .pdf of the complete game which includes setting materials. This book was for quick access, and generally serves well in that fashion. The game itself is excellent, and in reading through it it is well organized. However, there are a few tables that need to be quickly accessed, along with some reference materials (A page of costs for everything from buildings to meals, the weapon and armor tables, interactions for Companies), which are in their respective chapters, but not repeated elsewhere. If there was a short compilation of these tables and a quick reference for the Company rules, this would have been an easy 5 stars.