On hearing that there was some sort of offshoot fiction of Narnia, I was eager to read "just something" and worried that it might be terrible.  The titles seemed promising, albeit clear that they were taken from Lewis's books, and the books were less expensive than, say, buying new clothes.  The books are hard to track down;  many gamebook collectors have less than half of the set, and even when they are located, they are quite expensive.  Ten dollars was the best price I could find.  The book had several odd quirks like using section numbers instead of pages, as well as numbers to simulate dice-rolling in inner corners.
 The game action itself starts slowly;  many of the "choices" are rather transparent.  From page A you can go to B or C, both of which lead to D.  Ones like "Have you been to Narnia before?" are the most simplistic and leave the reader begging for a puzzle.  But the puzzles that are frequently moral choices(of the "Do you leave Jeremy to rot in a cave by himself?" variety) are transparent.  The puzzles, like much of the book, are filled with the sort of preaching that Lewis avoided, and you always wind up coming back to the main plot.  It's not bad, if predictable.  A warlock tries to raise the White Witch from the dead, and you may be put on the Stone Table and sacrificed if you are not careful.  Well, actually, there is only one ending, where you return to Narnia, although Aslan says different things to you before you go.  The main puzzles to solve here are moral, and there are only a few instances where you get clues.  One is at the banquet hall.
 Probably what's most annoying is how the author will either take passages almost verbatim from various Chronicles(LWW, PC, HHB, MN) or "dumb them down."  In especially inventive spurts he changes Lewis's third person to the second person.  Then, for variety's sake, he includes things that could not possibly be relevant to Narnia.  The two extremes do not average out in this case.  In one attempt at originality he mentions a sword given to King Edmund on the fifth anniversary of the fight at Anvard.  However, research into Lewis's writings would have shown that, in fact, Rabadash's treachery occurred in 1014, and in 1015 the Pevensies returned to England.  Many other attempts fail, as well.  Narrative asides are condescending.  The new names are interesting, but they make one wince a bit.  One might argue that the book is "simple" which has the virtue that you can easily reject it or place its primary value as a collector's item.  I guess you couldn't really expect a Lewis-like performance from a commissioned writer.