want to ask if they can recommend other people for you to talk with to gather more information.
the information you gain from these interviews is like gold! people who are working in your field will be able to tell you things you just can't find in any published resource.
it is also the case that if they have taken the time to speak with you, they are going to want to be helpful. they are going to want to feel like it was worth their time. they will want to know that they have helped contribute to the success of someone else. 