I've been using Game Maker for some years now (since version 4.0 (it's currently on version 6ish)), although I did stop using the development tool due to being busy in other aspects of my life.
I got to the stage in learning where I was capable of creating games of some stature, but never managed to get around to doing it.
There are some good points about Game Maker and some bad ones. I think that the book that Mark Overmars has written here should be a wonderous help to all those "n00bs" (as they get called at the Game Maker Community Forums (not always the friendliest places)). I do hope it becomes popular, because I'm sure it will aid new-comers in the development process of Game Maker.
I myself have moved away from Game Maker and have recently been looking at several alternatives (the main being using C++). I know I'll probably go back to Game Maker, perhaps when the new version comes out (and now Mark has finished the book he can work on the new version!), but I don't feel that I need this right now.
If the book is marketed correctly, aimed at the right readership (which tends to scale from 10 year old boys to 40+ men), It should do well, and as long as the users are notified of the book before purchasing the registered version of Game Maker, I'm sure that many people will find it handy.
In summary, it's about time something like this was made, if only it had been made 3 years ago so that I wouldnt have spent 2 years trying to learn GML with multitudes of people shouting "READ THE FUCKING MANUAL!".