You woudn't want spikes; they couple rather than isolate the speaker. Before spending top dollar on specialized isolation stuff, try heavy-duty rubber or silicone stick-on feet from the hardware store. Might be all you need to keep the bookshelf from "singing along". And of course if the bookshelf is already full of books that helps a lot, too.
If reflections/diffraction from the top of the bookshelf are a concern try something like heavy felt or a piece of carpet covering the shelf instead of the feet. You could probably find a 'table runner' made of reasonably heavy fabric in the linens or tablewear section of a dept. store, or someplace like Pier One or Bed, Bath & Beyond that would be more stylish if that's important.
