You can easily increase the buyancy by adding something buyant to them like a couple of air, or better still; helium,-filled PET bottles underneath them or perhaps a Zeppelin. Also depends what medium you want them to float in?
Water and blood have about the same density but if you want them floating in air you might want to increase the ambient pressure by a few orders of magnitude and have a light-weight container with a vacuum or helium/hydrogen inside.
Mercury is great as a medium at the first glance because of its high density but its actually quite corrosive both to metals and other things. If your shelves are glass or plastic this can work.
Molten uranium is even denser but does create its own problems. If you can get around them then ideally you can create an equilibrium between the decay heat from U235 and the heat losses so you can maintain a steady state molten pool in your house. It can cover your heating needs for the rest of your life!
https://en.wikipedia.org/...i/Archimedes%27_principle