Well how much spit is there? Are the pages getting wrinkly because they’re drooling all over their fingers? That’s just rude because they’re damaging the book.
Who is this person exactly? Am I familiar with, and approve of their cooties? Then go right ahead and lick your fingers. If you are some random skeevy person with giant pustules and stuff, then No! A thousand times no!
With normal, approved, spit cooties, it is a fact that by the time the spit dries and you go to read the book yourself, the cooties have been neutralized by healthy book magic. Magazines do not have this magic and so devilsknew’s Hustler is permanently cooty-ized and the print equivalent of an untouchable.
With books, the stuff I worry about is underlining, spilling food and drink, dog ears, and spine breaking. Book magic is a slight, gentle magic and cannot repair these. If you do this to a friend-- erm, book I’ve loaned you… Well, they’ll never find the body, that’s what.