The spell Hardening would increase it's hardness. Nothing prevents repreated castings of the spell, though it can be dispelled.

An option for higher level wizards would simply be to PaO their spellbook into a dangerous creature. Even if the creature is slain, it simply reverts to a spellbook.

Traps are fairly cheap compared to other items. With the Craft Wonderous Items feat you can create any number of spell 'traps' set to trigger vs certain stimuli.

An intelligent Blessed Book that polymorphs into a Greater Shadow whenever it is in danger and fires off a Sending to the wizard as it does so would be pretty nifty.