The introduction explains the nature of this book clearly: a collection of ideas, plots and ancillary material to empower the creation of a fantasy espionage campaign using whatever ruleset you prefer and based in your own campaign world. You'll need a copy of Encryptopedia from the same author, which contains some system ideas that provide underlying structure to such a campaign which are referenced here.
To start with, the entire campaign revolves around three factions. Two are explained in Encryptopedia and referenced here, the third is a new one and explained in more detail. This third bunch, the Shadowhood, practises a discipline that is part martial art, part philosophy and part magic... and in essence become almost ninja-like, gaining a range of abilities through their studies. They also have as a mission the collection of mystical stones called 'Forever Diamonds' or 'aeon stones'. The other two are governmental agents and organised crime, by the way.
Next is a look at the campaign itself, described as a 'sandbox with open play', i.e. the party has free rein to do as it wills within the campaign world... but so does everyone else, and especially the three factions which each get on with their own plots and actions irrespective of what the characters are doing and whether or not the party is paying them any attention. This creates a nice 'real-world' feel in which the party remains the centre of attention, the stars of the show if you will, but life goes on around them regardless. Some characters are provided: they could be used as pre-generated PCs but the real intent is to provide a basis for filling out the factions that the party doesn't choose to join (all of them, if the party chooses an independent path). Within the constraints of a generic systemless concept, there is quite a lot of detail for each: it should be fairly easy to add appropriate mechanics for whatever system you are using. In essence, they are three teams that operate in a similar manner to a party of PCs, whether they become enemies, allies or a bit of both depends on how you decide to run the campaign.
The next section presents The Voyage of the Lucky Manticore, an adventure outline which provides a starting point for the campaign. There are three set-ups, based on which faction the PCs have decided to be a part of - if they haven't decided yet or want to be independent, you'll have to find your own way to embroil them in the action as it's presented as a mission from faction leadership in each case. Based on a pleasure boat - think floating casino or perhaps something like a Mississippi paddle-boat - there's ample mischief to be had... including a card game called Pakka. An abstraction is provided for groups who don't want to actually get the cards out, else just use standard poker rules.
There is very much a feeling of allowing the party to do what they want aboard, with a timeline of events to use as a backdrop to whatever they choose to get up to. This includes highlighted notes on the actions of key NPCs. It is suggested that you read the player notes for all three factions so as to devise additional events around what the others want to accomplish... or of course if your players haven't chosen a faction yet, these events might embroil them in the action regardless and lead to them becoming involved.
The next section introduces the Campaign Engine. This includes both mission generation and an escalation matrix. Sometimes the party will work out what they want to do on their own, at other times their faction will give them missions to undertake... and all the time, the other factions will be circling around, watching, doing their own thing and interfering with what the party is trying to accomplish. There are different types of mission: acquisition, research, incursion/interdiction (the party interferes with another faction's operations), and mandatory override missions when something crops up that must be attended to RIGHT NOW irrespective of what else is going on. You'll need to refer to Encryptopedia at places, it's got loads more on the sorts of things that can be brought in to your plots. The escalation matrix enables you to manipulate and track the interactions between the factions as the plot proceeds. These relationships can vary from an alliance to outright open warfare on the streets!
Finally there is a section on campaign milestones, giving ideas on how to structure the plot over and above a series of missions by providing for a beginning, middle and end. Some extensive background is also provided, as well as an appendix giving details of the aeon gems or 'Forever Diamonds' whose acquisition forms the central feature of the campaign.
Whether you want to build an entire campaign around espionage or want to introduce elements of the conflicts it generates to an existing campaign, there are plenty of ideas to get you going. Perhaps relatively low-level characters arrive in the area where all this is going on, and are of sufficient potential for one (or more!) of the factions to approach them and make an offer for their services. Now I have been known to introduce elements of 'conventional' espionage (that between nation states) into my fantasy games, but this puts a whole new exciting spin on things. The concepts will fit in with any game mechanics, and if you want to use a published setting, most have appropriate places in which to locate your adventures. This isn't a campaign ready-to-run, it's a wealth of ideas and resources to aid you in building your own epic.