What Root and Neal mean by "imagination" is that Lewis speaks to the heart as well as to the head. Most of us are accustomed to Lewis' strong logical apologetic in works like The Problem of Pain or Mere Christianity, but it's Lewis' desire to speak to that part of being human that is "older" and more nascent: The human heart. Imagination isn't simply make-believe. Just as airline pilots nuance their training with experience in air currents, so Lewis believes we humans have something much more important than simply our "Reason". That part of us is what Lewis calls in various (albeit scattered) places the "imagination".
And Lewis finds the reconciliation of that reason and imagination in "the mind of Christ". For Lewis, imagination is as much a source for truth as reason. Because imagination allows us to perceive and experience as humans that reason alone cannot.
Imagination, in short, helps expand our world.
And while Root and Neal look at some of Lewis' works that most of us will never read -- particularly Lewis' academic writing like Studies in Words or Preface to Paradise Lost -- they continually put before us the notion that Lewis himself insisted upon: "The imaginative man is older in me than the [rational man] and more continuously operative".
The Surprising Imagination of C.S. Lewis is not only a wonderful fresh look at Lewis' thinking, but it's also a great introductory text to some of Lewis' most important writing.
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I received a free copy of this book from the publisher for my honest review here.