Roland writes:

another option is to rebate the back of the shelves into the backing of
the bookcase, you will lose the adjustability but you say you will be
using the bookcase for textbooks - a known height. rebate 1/8 of an inch
1nto say 1/2 inch ply backing? you should gain a lot of strength and
stiffness

Well, I was using the term "textbook" pretty loosely plus suprisingly
my textbooks do vary a fair bit in height