How to remove margin notes space in book class?

I would say that the book class (nor any others) does not saves space for margin notes; rather, it allows you to specify the maximum space that you want to allow for marginalia so that they do not exceed the specified margin widths.

In two sided printing (e.g., for books) the basic layout is a double spread --- left hand and right hand pages open where the width of the outer margins are equal and the apparent width of the gutter (the distance between the left hand and right hand textblocks) is the same, i.e, all three white columns are the same width.

See any good book on typography; Robert Bringhurst's `The Elements of Typographic Style' is one of the best.