You get a lot of answers because there is no such thing as decoupling. There is only vibration control. This being the case every bookshelf/speaker combination vibrates differently and so there is no one universal answer. If decoupling was a thing then it would always work. So you can be sure its not a thing.
Coupling usually works, because mass is one of the easiest ways of controlling vibration. This is also why sand boxes and massive stands tend to work so well. You could even sandwich a speaker between two books, a thin one under and a thick heavy one on top. Not saying this will be great, but it is something cheap and easy you can try just to see what I mean. Ultimately it all comes down to trying a few things and going with whatever works best in your situation.
Coupling usually works, because mass is one of the easiest ways of controlling vibration. This is also why sand boxes and massive stands tend to work so well. You could even sandwich a speaker between two books, a thin one under and a thick heavy one on top. Not saying this will be great, but it is something cheap and easy you can try just to see what I mean. Ultimately it all comes down to trying a few things and going with whatever works best in your situation.