Your thought about using cable ties is interesting, but I wonder how you would attach the ties to the furniture without screws. I do agree that shims under the front side will stablize a bookshelf for the most part. If your bookshelf is open back you could attach a cleat to the wall just under the top for a more stable usage. A cleat could be just a 1 x 2 piece of wood cut to the inside measurement of the top opening and attached to the wall. The shelf would ride on top of the cleat and the lenght would keep the shelf from moving side to side.
You can use some sort of safety cleat, like those used to mount appliances and tool cabinets. I don't see how you can avoid fastening it too at least the backside of the furniture somehow.
I guess you could use some sort of strap or pallet banding system that would run through a bracket or two anchored in the wall in back of the furniture but won't the strapping running around the bookcase look unsightly?
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