Well, I use tinfoil or a sealed metal tin to stop things becoming accidental portals, things like Tarot cards, books, other devices that are some kind of interface with “the spirit world” to use a hokey expression, I store inside metal. Tinfoil is very convenient and cheap, metal tins that seal nicely also work well.
I do a lot of weird shit and it just makes sense, it doesn’t mean you’re paranoid if you lock your front door at night.
In the case of hauntings or unwanted poltergeist stuff, anything like that, wrapping everything in metal foil/putting it in a tin is the first thing I recommend.
It’s possible the energies from the books got disturbed by the theft and need to be reassured and placated, so taking them outside, ideally placing them on grass or bare soil, offering them a drink (one for each tome) and maybe some tobacco or incense, ring a bell if he has one, and assure them the theft is over and they’re back under his hand.
Treat the books themselves like high-spirited racehorses that got spooked and need to be calmed, taken out in nature a bit and shown things are back as they should be. I find spirits of objects in general respond well to being treated like this.
He may be able to feel an energy shift take place, and then bring them back inside after ideally a couple of hours outdoors (unless he really can’t do that for fear of more thefts and whatever, or weather, or slugs), wrap them in foil/put them in a tin, and then see how things go.
This is what I’d do in this situation. If he can offer a little cup of beer or spirits, once the books have been on the ground a while, pour that just in front of each one, and state (aloud or in his mind) “I offer you (book’s name) this drink to mark your safe homecoming.”
Objects have kind of an aura and pouring the drink near the book will allow it to pass through as it becomes “expended” - I mean, passes the point where anyone else could have it.
Norse shrines consisting of rocks used to be given offerings of blood and alcohol by having it poured on or near them and I think that may have been for the same reason - with a drink the true sacrifice comes when you willingly give it so that it can’t be recovered. At that point it becomes “killed” and its purpose as a drink is lost, equivalent to sacrificing a living being.
Don’t give the books blood, though, they’re already probably a bit spooked and acting up, they need to calm down a bit. If he’s a stoner, blowing some smoke over them would probably help, anything calming like that.
These threads may also be useful to him: