It would kind of bother me. Even if it were my boyfriend, with whom I swap spit at every opportunity. I would ask whoever it was to stop doing it, and I might think twice about handing them a book in the future.
It’s not exactly rational, but I’m very protective of my books–even the ones I don’t like and probably won’t read again and might even sell. I can do whatever I like to them (and I rarely treat them with any extra care myself), but as long as they belong to me other people had damned well better respect them.
On the other hand, they could actually spit on my furniture or dishes, and while I would think it odd that they had done that, as long as they cleaned it up properly I wouldn’t be put out in the least.