If your book shelves are adjustable (removable), when you see them start to bow under the weight of books, remove the shelf and reinsert it upside down. Then replace your books. This way the weight of the books will flatten out the bow.

If the shelves in your book case are fixed and non-removable, and bowing under the weight of the books, here's a suggestion... cut 3/4" plywood panels that are the height of each area between the shelves, and insert them in the center of every shelf opening starting at the bottom. This way, the weight load of all the shelves is supported in the center by the base of the shelves or the floor.

The 3/4 in plywood inserts take up the space of a book and can be stained or veneered to make them look attractive. Some enterprising numismatist might go to a local library book sale and purchase dummy books of the right size where the spine could be glued to the plywood, or the entire cover could be wrapped around the plywood.

There is another problem I've experienced that can be addressed... Have the adjustable metal clips (or metal pins) that support shelves ever collapsed under the weight? If so, these same plywood panels could be placed at either end of the shelf instead of using the clips or pins. My wife's three glass door china cabinets have experienced both the bowing shelves and the collapsed shelves.