Glyph of warding
A glyph of warding placed on an object can hold a spell. A fireball will burn up a flammable object, such as a spellbook, in its area of effect. The PCs could theoretically defeat the glyph by dispelling it, and they might be able to detect it with an Investigation check, as described in the glyph of warding spell description. Compared to contingency, this has the advantage of not taking up the wizard's single contingency option. It has the disadvantage of requiring the warded object to remain in one location, but that might not be a problem for this wizard.
Leomund's secret chest
A wizard doesn't need his spellbook to cast spells, only to change the ones he has prepared (or cast rituals). A wizard can safely store his spellbook, or a variety of other valuable items, on the Ethereal Plane using Leomund's secret chest. Only the wizard who owns the chest can find it, so this is reasonably secure. If he dies, the chest and its contents aren't technically destroyed, but they're going to be awfully hard to track down.
Drawmij's instant summons
A similar effect can be achieved with Drawmij's instant summons, which allows for faster recall, but with a higher cost and less overall convenience. Since only the wizard who cast the spell can use the key, if the wizard dies, the protected item remains hidden (say, buried under a few hundred tons of granite).
A semi-mundane device
Some wizards might find it amusing to protect their spellbook with some sort of mechanical trap in their lair. Suppose a clockwork mechanism triggers a mechanical shredder that will destroy the spellbook if it's not wound every day - and the entire apparatus is thoroughly hidden (possibly accessible only via magical means of some sort). The wizard who inhabits the lair can easily wind the clockwork, but by the time any invader figures out what they need to do, it would be too late.
"A wizard did it"
Since the wizard in question has unusual spells, he could presumably have a spell designed just for this purpose. Your creativity is the limit, but what comes to mind is hiding the master copy of the spellbook in a place where only the owner knows where it is, and then having a spell to summon an illusory copy on demand.