This is probably one of the most helpfull gaming guides for any game ever, and because it's an official game guide it dosn't count as cheating!
This guide gives you every last spec of detail of the games different locations, items, weapons the lot. And you'll need it because to be honest there are many parts in the game where you'd think to your self "what the hell am I supposed to do here?"
The main walkthrough tells you how to complete an area in great detail, plus tips on how to get the best items during that mission etc. Another great feature is this time the optional side quests are incorporated into the walkthrough in their specific places, which means no more flicking back and fourth from the seperate chapters, making it much less confusing.
Also if you think reading your way through a detailed walkthrough might give away parts of the story before you get to them in the game... Well you'd be right ,but if you worry about that kind of thing at the bottom of each section theres also a shortend "runthrough" only giving info on what you need to do in a level and doesnt give any of the good bits away.
So all in all a great and vital walkthrough for the game its a must have. (unless you dont actually plan on buying the game)