Many reviewers in Stereophile mention using BluTak to anchor the speakers to the stands. I think I have seen it used to hold posters to walls without taks. But those are people that are taking gear in and out all the time.
If I had bookshelfs I would use four wood screws. Yes, you'd damage the bottom of the speaker........ but it is afterall the bottom of the speaker. Sure, bolts would be the most secure, but then you'd need to open up the speaker to really get in there........
So, I guess the options are.........
Nothing, just the rubber bumpers that do tend to stick.
And adhesive, like blue tak, or even some dabs of silicone caulk.
Wood screws up through the bottom.....
And nuts and bolts.