Ford Harding has some of the best sales books I have ever seen for those in professional services and consulting. I find that many other books in the marketplace focus either too much on product-oriented businesses or sales attitude to the exclusion of understanding why certain sales and marketing processes work for some business situations but not for others.

Ford's books are especially a must-read for management consultants with traditional firms and independents. Ford recently started a blog, and he's got an excellent post on cross markets and cross selling here.