The book's written like a novel with the walkthrough highlighted in yellow to separate it from the story. Since it's written like a novel (third person), the text aren't exactly instructional although you can always guess what to do from the text.
If you're using this as a quick reference when you get stuck, locating where you are might be tough as you're sifting through a chunk of text.
There aren't any maps included so locating objects, or yourself, might be a bit tough. The lack of maps makes you rely on the walkthrough which isn't spoiler-free, unfortunately. If you don't want to miss out items, you have to read in advance and sometimes you'll be smacked with a plot turn you wish you learn from the game instead of the guide.
Overall, it's a comprehensive guide that covers everything it should. The presentation style is definitely not for everyone.
In my case, I would love to play the story than to read the story.
By the way, there's a novelization of the game itself simply (and similarly) titled Alan Wake.
This book is also available as part of a bundle with the Alan Wake Collector's Edition Bundle, which includes a making-of book for the game, titled Alan Wake Illuminated.
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