Importance of interactivity in learning
You can attack it from many different angles.

Perhaps a good place to start is the distinction between the behaviourist and cognitive learning theories. Although both can be facilitated using interaction, behaviourist theories put the teacher as the focal point, whereas cognitive ones put the learner first. In that respect, a good interactive learning design will give learners more control over the learning experience. I also believe you can demonstrate that interactive learning has greater potential to be mapped to experiential learning (see Kolb Cycle for instance).

Another related topic is that interactive learning based on self-exploration is likely to result in higher intrinsic motivation compared to guided learning.

Another line of argument can many aspects of interaction that can aid cognition (or reduce cognitive load, if you wish) - A book (non interactive) is a get-it-or-leave-it type of delivery; interactive systems enable techniques such as progressive disclosure or contextual help which can markedly improve comprehension.

Perhaps important to assert that interactive learning, on its own, does not secure better comprehension - the experience design is what really matters; interactive learning can simply enable features that non-interactive learning cannot.

You should be able to find some research papers on the topic via Google Scholar.

These are just a few leads.