What are the differences between Adult and Kid versions of The Chronicles of Narnia?

An Amazon reviewer has looked into this and reported their findings:

After at least 40 minutes of Googling, I finally found out what the difference between the "adult" version and the regular version is. Apparently the "adult" version includes some essay material about the literature and each book contains a synopsis of information you'd need to know from the other books to read the one you're holding. Other than that, only the packaging is different. The stories all remain the same. I only wish Amazon.com would have provided me this information and saved me the time.

Doing a bit of further digging, it seems that the addition essay is one by C.S Lewis, entitled "On Three Ways of Writing for Children."

Various message boards have discussed this, and the consensus seems to be the same: there is no textual difference between the versions. The "adult" version, however has fewer illustrations and a "plainer" set of covers.