Make tests to prove the value of each program. Drop those that do not prove out after a reasonable test. These can be purchased at most any store, on the corner found at the library, or bought through the large Mail Order book clubs. Scientific, technical and text books are other categories to stay away from. All others have a good Mail Order potential.
Sex books are one of the most profitable fields of selling books by mail.
Repeat orders are high, but so is the penalty if state or local laws are broken. The Publisher does not advocate selling this kind of material as one can make a fortune without resorting to what is or could be considered immoral. Mystic and metaphysical books are more or less specialized field ad generally the demand is not large. Health and recreation books have a steady market, but they are mostly tied up by firms in the catalog business. Choose the type of books that interest you most.