Placer Gold Recovery Methods - Michael Silva - This is a great little book put out by the California Division of Mines in the 1980's and now back in print by Sylvanite Publishing. It covers in detail all the methods used to recover placer gold such as Panning, Sluicing, Jigs, Centrifugal bowls such as the Knelson Concentrator, Belt Separators, Reichert Cones, Helixes, Shaking Tables and more. These are still the standard machines used today in most placer operations.