Photoshop :: Cropping And Straightening

i have photographed an old book of mine, and i am batch cropping and straightening the photos in photoshop, but the cropping fluctuates from, in some shots, editing out just the borders around the book, which is what i want, to editing out one whole page (of the two per photograph)- something i do not want. there does not seem to be any logic to what is edited out or not, it seems random. can anybody help with this inconsistency problem?

i want to edit out just the border (background) around the book, but leave both pages in the photograph. why at times is the program editing out whole single pages leaving only one page per photograph?

P.S. just to update, i have realised that perhaps the problem has to do with the fact that the two pages on the book have, like all books, a dark line vertically down the centre between both pages where the binding is. think of a photcopied page which has a dark line down the centre separating left from right page. i believe that photoshop reads that black centre-line as a border itself and so edits one page out; it assumes the black line is a border and edits out everything outside of it (which contains the other page).

now i'm not sure how to overcome this. is there a way i can set it so that the cropping feature works on some rough template to do with size? the page sizes are relatively similar, so if it used a size-type template to crop, it would work well. this problem is random and not all photographs are problematic, but quite a few seem to cut out one whole page.