You can find the full article with the annotations as to why these questions are important here.
Jim starts out by saying the following:
He suggests asking the following questions:
(be sure to read the full article for all his reason's WHY you should ask these questions)
What about my book did you respond to?
Do you have editorial feedback?
What’s your editorial style?
How long have you been with your agency?
How many clients do you have?
What is your typical response time to email/phone calls?
How do you like to communicate (email vs. phone)? And how often do you communicate during a submission?
What happens if you don’t sell this book?
How many editors do you go to before giving up?
What percentage of projects that you sign do you sell?
How long is your average client relationship?
Who do you work with to sell foreign/film rights? What is the agency’s support staff? Do you handle contracts? Rights? If not, who does?
What does your agency agreement look like?
Can I speak to one or two of your clients about their experiences working with you?
[Specific to any author who wants to work in more than one genre]: Are you open to authors who work in multiple genres? And are there genres/age groups/etc. that you don’t represent? If I write something in one of those, what then happens?
And lastly, he shares a very brief what NOT to ask:
How much can you sell my book for? How long will it take to sell my book?