I have been obsessing over Lewis for the last year and a half after re-reading the entire Chronicles from start to finish, something I don't think I've ever done. (I've read them all at least a half dozen times or more, but never back-to-back.)
The inconsistencies in the Chronicles, the sloppy writing, the bizarre references - I just HAD to know what was up with Lewis. Consequently, I've been reading as much as I can about the Chronicles and Lewis himself. My theory is that he was so intent on getting his message across that he didn't pay attention to things like plot, structure, continuity, etc.
"The Keys to the Chronicles" was absolutely critical in helping me understand what on earth was going on in Lewis's head. If you have ever said, "WTF is he doing?" read this book!
I read this book at the same time as I read "The Way Into Narnia" by Peter J. Schakel. I actually would read about LWW in one, then LWW in the other. Individually, they answered a lot of questions, but together, they offered nearly a complete explanation of everything I've ever puzzled over.
The only thing I didn't like was that I didn't know who that girl on the cover was supposed to be; she kind of looks like Amy Pond. I loathe Amy Pond, so I decided she must be the White Witch.