It depends on which part of sales you want to focus on. I've yet to find a good book which covers the discipline as a whole. Many seem to either focus on the psychology, prospecting or just closing of sales.

I'm part way through "New Sales. Simplified" which seems heavily based on "prospecting" which a lot of books seem to neglect. I've also got Dan Pink's "To Sell is Human" lined up next which is more about the psychological aspects. Many of the classic sales books by Zig Ziglar or Brian Tracy seem, IMO, to be more closing focused.