Start in the front and work your way to the back carefully tearing out 3/4 of the pages one page at a time. I leave pages where the art, photos, or captions are intriguing and might inspire future art journaling. I choose books which have some interest as the words and images will be incorporated into future artsy reflection. So, for me, a geometry or physics book just won't do! (Not a point of interest or inspiration for me!) I also like thick pages so they can stand up to the wear and tear of the art.
This one is "Pocket Gardens" a book I ordered on line that was disappointing but not worth returning. It will be put to great use now as a mixed media altered book journal.