CategoryBook: Crossing the Chasm : Marketing and Selling High-Tech Products to Mainstream Customers by GeoffreyMoore,
Pretty much essential reading for any commercial software designers. Should temper this book with GuyKawasaki's RulesForRevolutionaries.
See also http://www.isa.org/~setex/pdffiles/982011.pdf
A sequel to this book by the same author is InsideTheTornado.