The guide is formatted as follows.
The story so far
A brief summary of the pre-mission cutscene, as well as any other pertinent info. Will usually contain spoilers; only really big ones will be marked.
Information on the target, if you're so inclined.
Weapons you can find in this level and bring back to the hideout.
Any information that you'll need to go it alone on this mission.
A full walkthrough for the mission with a Silent Assassin rating; i.e., kill only the targets and hopefully make it look like an accident, without witnesses or being caught on tape.