Drawmij's instant summons is useful as an anti-theft mechanism for high-level wizards. If the book is lost or stolen, re-summon it to your hands. It won't return if held or carried by another creature, but you at least know their location (and can then teleport in to give them a spanking.
Bind the book with mithral or adamantine covers, and attach an adamantine lock. Cast arcane lock to make it extra-secure. As written, the target isn't a door, window, gate or chest... but it's a harsh DM who wouldn't allow it to be cast on a padlock.
Leomund's secret chest, to store the book on another plane. A liberal DM might even allow this to combo with glyph of warding. Cast the glyph on the book, put it in the chest, send it off-plane. Re-summon the chest as needed. The book leaves the plane, sure... but does it move more than 10' from it's starting position on the Material Plane? Arguably, it's distance can only be measured in relation to the chest in which it is contained, and it never moves more than 10' from the chest. So, the glyph remains active the whole time. Having said all that... frankly, I'd allow the explosive runes variant of the glyph to remain active on a book even if it is moved more than 10', regardless of secret chest shenanigans.
Nystul's magic aura, if you're worried about an enemy caster locating your spellbook via magic.
Planar binding, commanding the bound creature to attack anything that touches or opens the book other than yourself. Works best on some creature that can actually occupy your book! My creature of choice is a yochlol demon. The yochlol enter mist-form and seeps into the pages of the book. Unsuspecting fool opens book and a toxic mist envelops them. It gets worse from there...
Programmed illusion. Too many uses to even list. Summoning up an illusory demon makes it look as if you used a planar binding to protect your book, lasts longer, and costs only a fraction of the component price.
Sequester can make your book invisible. Difficult to read, of course... but if you're high enough level to protect your book with sequester, you're high enough to use true seeing so that only you can read it.