Johan, it depends of the context. Shell voicings (http://www.thejazzresource.com/shell_voicings.html for example) in the left hand are useful when you are playing by yourself. They provide the harmonic foundation for the melody played by the right hand, and eventually, for improvisations. Being able to play a tune out of a fake book by yourself is a basic skill for you to learn, and developing ease in using shell voicings is part of this skill. Rootless voicings, which are used both in solo and ensemble playing, are usually learned after one develops facility with shell voicings. I recommended The Jazz Piano Book because both are covered there. Improvising Jazz Piano by Richards is a more basic book that develops important skills and knowledge. I don't think you could go wrong by going through both volumes of Improvising Jazz Piano and using The Jazz Piano Book somewhere along the way.

I also suggest you check out a series of solo piano books of arrangements by Brent Edstrom. These contain sophisticated but very playable solo piano arrangements which provide great examples of what solo piano should sound like. See http://www.amazon.com/Jazz-Lovers-Pi...9SV5BZMP5WF3FN.