A typical bookshelf carries a load of approximately 25 lbs per linear foot (~11 kg per linear 300 mm). Den is right to mention timber type being a factor in the load carrying capacity of a shelf, and shelves between 600 and 900 mm long are common enough where the thickness is 18- 25 mm, whatever the material used, even chipboard, but at this length chipboard will sag very visibly. Changing the depth (thickness) of a beam (shelf) has, perhaps, the greatest impact on the ability to carry a load as the increase in strength is a square of the depth. For instance, doubling the depth of the beam means it can carry four times the load. Instead of making a shelf that is twice the thickness you can get essentially the same effect by adding lipping at the front and/or back edges of a thin shelf that is twice that shelf's thickness. In other words, if you add 36 mm deep lipping front and back to an 18 mm thick shelf you'll just about quadruple the book carrying ability.
I'll not go into detail about creep deformation which is permanent sagging due to the length of time a beam must carry a load, where this permanent bend, even when the shelf is unloaded, shows plastic deformation and indicates at least some failure of the shelf's structural make-up, e.g., wood, plastic, metal, chipboard, etc. Slainte.
