The way I see it (Which is totally just a personal opinion that matters nothing, but hey, you asked for me to explain in Other) is that this is paying for information.
Thanks, that was very valuable to me.
Be assured that the book contains flavour material and some background information on what's going on inside the Blight, but none of that is relevant towards surviving the 5th invasion. You can - and are welcome to do so - consider all the material IC knowledge if your character can explain how he got those letters, reports, etc. - and that is why I've published it now and not after the invasion. First, it was finished now and I didn't want it gathering dust and two you can if you want incorporate it into the current game. But that only gives you stories and atmosphere, it doesn't give you strategically important clues.(*)
(*) unless "Daimons are evil" is something that for some reason is news to your character.