Wherever possible the width of the type area should come out even in full picas, or in half picas if absolutely necessary, and the inner margin or gutter at least in half picas again. (learning.north.londonmet.ac.uk)

When printing books with bleeds that run into the binding margin, as with art or photography books, please leave 3/32" unprinted and unvarnished glue trap in the gutter between signatures. (Acme Bookbinding)

Finally, most books lose a little of the readable page to the gutter, the inside margin. (Lulu, Inc.)