As you state, preferred shares act more like bonds. They have a set dividend rate based on an issue price (usually $25 or $50). Upon expiration or if they get called, you'll get the $25 or $50, so never pay more than that.
They are "preferred" because they get paid dividends and upon bankruptcy before common stock, but after bonds. Because they are equity, the dividneds may be taxed as qualified dividend income, and not ordinary income like bonds. That is not the case for all preferred shares.
Most preferred shares are in financials and utilities, with a few REITs thrown in. To my knowledge, no one has created an "index" of preferred shares. The are some ETFs out there. See
http://etfdb.com/type/preferred-stock/all/. You can compare holdings, fees, etc.