For writers who use any sort of violence in their stories, this book is hands down an excellent resource. Regardless of what time period you write about, regardless of whether your character is on the receiving end of violence or whether s/he should know how to fight (in any way, shape or form), and regardless of whether you yourself have had any kind of weapons training, martial arts, or self-defense course, I would highly recommend this book -- especially if you haven't had any personal experience with violence in real life (or even if you have but haven't had experience with the type of violence your character encounters or uses).
In this book, Rory discusses the physical aspects of violence and the logistics of it, which are essential to know when thinking about how a character would realistically fight in a given setting. He also expounds upon the levels of violence and the interaction between those levels, which is helpful in measuring what level of violence your characters fall under and are comfortable with (and which ones they're not). He describes various types of violence, so you'll understand exactly what type of violence your character is involved in, and he shows the motivations and mentalities involved in perpetrating those types of violence, which will help when giving your character a reason for entering into violence (as well as a reason for avoiding it if at all possible). He also explains what types of mentalities are needed for the use of particular weapons, which is a factor in deciding what kind of weapon your character's personality type would be likely to choose and wield. And it could present some interesting conflicts, too, if the character is forced to wield a weapon s/he has problems using!
He doesn't leave out cultural or social aspects of violence, either, including war and varying outlooks in different cultures. This is great when giving a group or an individual motivation (and justification) for killing in wartime AND in peacetime, depending on the attitudes of a particular culture.
Also (and this I gather is his real motivation for writing the book), he deflates many of the common tropes used in action films and books today, which form the extent of many writers' knowledge of violence. Unfortunately, when used as a basis for fight scenes in books, it may leave them sounding unrealistic and unintentionally slapstick. If for no other reason than that you don't want people rolling their eyes at your kick-butt character's awesome fight scenes, you should really check this book out (It's why I did!).
The writing itself is also easy to read, almost as if you can hear him talking, filled with fascinating (at least for me) facts, and I even laughed out loud a few times (and, yes, there are a few typos but honestly? I'm reading the book for information, not to mark off points for bad editing). Moreover, Rory is open and honest about what he has experience with and what he doesn't, and he provides practical advice on how to find out some things for yourself (for example, if you want to find out what different types of sword swings / thrusts, etc. will do to a person's body, try it on a sausage - not necessarily an appealing afternoon's activity, but it's a better alternative to doing a Google search that might get you on various security agencies' watch lists!).
Additionally, he delves into gender differences (behaviors, views on violence, etc.), different types of weapons (people themselves, guns, knives, swords, axes, blunt weapons, less lethal weapons such as pepper spray...), habits of professionals, relationships between professionals and civilians, what a professional would look for (and at) when entering a potentially violent situation (or even just when entering a room), and what s/he would notice. He writes about the types of wounds received by certain weapons, what parts of the body professionals proficient in certain types of weapons go for, what people experience when they're in the middle of a violent situation, what they feel (and what they don't feel), what they hear (or don't), how certain wounds feel (or don't feel)...
This is just what comes to mind off the top of my head.
In summary, I would definitely recommend this book as an excellent resource for anyone writing anything pertaining to violence (or also, I would venture to say, for anyone wanting to understand the nature of violence in and of itself, because for those who don't understand it, this book can be a real eye-opener).