This thread got me thinking of ways to guard a spellbook cheap and easy, and i came up with this:
Take one spellbook, one Alchemical tooth (CAdv.) and one casting of shrink item.
Prep spells, shink book into a tiny cloth stamp, roll it up inside the tooth and stick the tooth in your mouth.
Book is only in the open whilst preppin, it takes a DC 30 Search check and more importantly rooting through your mouth to retrieve it. It's an enclosed container inside your mouth so theres no line of effect for Dispels, Disjunctions or Anti-Magic Fields*.
[Retrieve item] is not a stated property of Wish (unless there is a spell with the function that it could emulate), so no threat of stealthy retrieval by Wish**.
Since it does involve pulling out a tooth, NPC wizards are highly unlikely to consider it, since their books are not at risk anywhere near as often as those of adventuring wizards, so the trick is unlikely to be widely known and prepared for.
Final pre-emptive counter: if a PC had a cursed arrowhead/similar object-in-wound impairment, would you remove it if they wandered into one of those item/owner separation teleport traps? Or would you more likely say that the item in question remains since it is inside their body?
With that done, can anyone point out a reasonable scenario where a wizard who has done these preparations would lose their book?
And if no such scenario exists, can we just agree to deduct the 1 tooth, 300 gp and 1 3rd lvl spell known/slot-per-day as "Spellbook Protection Money" and call it a day ?
*A Dead Magic zone would most likely pop it open, so have someone else take the lead with a sealed Eternal Flame lantern (lantern goes dark-> DM zone -> go somewhere else. And a "roaming" DM zone is pretty much the DM out to get you.
**Technically possible if DM okays it, but at that point every single non-magical item in the campaing world just became instantly retrievable by Wish. Unlikely to happen and again toes the line of DM-fiat gimping the wizard.
Edit: as additional precautions, only keep the tooth in mouth whilst sleeping in case of pit traps into Dead Magic Zones, keep an Oil of Make Whole on hand (in case of sunder), keep a fake spellbook (Read Magic + scribbles with Illusionary Page) in haversack and prep spells inside ropetrick/similar (everyone else outside).