Marcille and younger Laios did specify "memories he hasn't taken yet," and Al can tell the difference between the book (untaken) and the other things he's touched (either full memories that must already have been recovered, infodumps of things Laios is pondering, or a mix), so he can probably figure out that the tombstone needs to stay where it is...

...or he can try something wild? I'm just narrating here.