This book is a seminal piece.
Rackham set out to determine what kind of questions the best salespeople used to get a sale done. What he discovered was that the best all asked specific types of questions in in a specific order.
S - Situation Questions : What is going on?
P - Problem Questions: Identify the problem and have client expand on the pain.
I - Implication Questions: What are the implications of this problem persisting?
N - Need-Payoff Questions: What would help make that problem go away?
This system has been taught for years. If you are interested in sales, you must read this at some point.
Please feel free to comment on any of these recommendations and suggest other sources that might be helpful.