Torsion boxes are great at increasing the stiffness to weight ratio compared to solid wood
but they are not stiffer than solid wood. At one inch thick you'll need to pay attention to
grain direction to get the most stiffness and maybe use a stiffer species and cover it with
the typical veneer. Increasing the thickness of the shelf whether solid or constructed is
the best way to increase stiffness.
Wood doesn't come straight, we spend time making it straight and you could make it
bowed up but when it is pushed down it will exert outward pressure on the sides.
Still 60" is twice as long as is recommended for ordinary shelves.
Encyclopedias may require something like JMB suggested.
A steel tube is a steel torsion box ready made, but it too can and will bend.
A feature of steel is that if you bend it enough it will stay bent yet still retain elasticity.
You could make a laminated shelf (tortion box style) with one or two pre-bent steel tubes
(3/4" or 7/8") in the front half and straight wood strips in the back half. Use epoxy.
When un-clamped the front of the shelf will spring up and ideally the full load of books
will be required to bend it straight. Even if the cabinet back is only 1/4" be sure to fasten
it to the back of the shelf. That will greatly increase stiffness but it won't be adequate to
keep the front straight unless you take some of the extraordinary measures that
have been suggested.
Your client needs to understand that at 60" a simple straight shelf when loaded will sag and
an engineered shelf when unloaded will be crowned but will straighten under load.
