The golden fleece of book design has been the typesetting of the typed area in relation to the paper and the spread. Famous typographers such as Jan Tschichold and Rosarivo devised canon's of page construction detailing how to calculate margins and set the typed area.
Having looked at over 100 books as part of some development work I am trying to complete, the most immediate and apparent observation is that most Publishers and obviously their book interior designers currently disregard such rules. My own feeling is that this is mostly done for monetary reasons. The books in my opinion still look very good and maybe is time to let medieval rules fall away.