I had asked the same question a while back. Several recommended the book Turbine Pilot's Flight Manual by Brown and Holt, which I purchased via Amazon ($30). Be sure to get the second edition (2001). It has an excellent chapter on Turbine Engines and Propeller Systems, which is geared to former military jet guys (me) and pilots who have flown only pistons. Good info on how turboprops work, although not all that much on what a pilot in the cockpit actually does with the levers and gauges. This message board and experienced T-prop drivers can help with that. The book has a lot of other good stuff too, and it would be especially helpful to pilots climbing up the commercial aviation mountain. So I'd recommend that approach.