As a C.S. Lewis fan and Evangelical Christian I found Clark's book a great overview and summary for the works I have enjoyed as well as works I have not yet read, such as Lewis' Sci-Fi trilogy. I appreciated the list of references in the front of the book and still have it marked for reference. It is an extremely easy read and flows well.
Clark's organization of the book worked for me. The layout helped me prioritize which Lewis works I will read next and what I will pay attention to. This book has helped me obtain a great overview of how all these works tie together, some links between the various works themselves, some how they link to events in Lewis' life, and most importantly how they link to how God "may" work.
"Lewis and Clark" have been used to really solidify my beliefs as a Christian; Provided logical, viable "answers" (that suit me) to many of the very difficult life questions we all must answer in some way. Ironically, just as the historical explorers Lewis and Clark opened up new frontiers for exploration and documented the "unknown" so have C.S. Lewis and David G. Clark.