Which is MCO book by Walter My mom is going to chess classes for some years now. Am absolutely noob. She has been asked to buy a book: "Modern Chess Openings" by Walter Korn. While googling, I came to know that the author of this title is changed over edition.
Till MCO12, it seems that it was completely written by Walter:



MCO13 says "Revised by Nick De Firman, under the editorship of Walter":


Now MCO14, 15 completely erases Walter's name from the book:

 


Is the content of the book considerably changed over these editions. Which one should I prefer?
The MCO series is styled like an encyclopedia. Just how encyclopedia's tended not to change all that much from one year to the next, the MCO series hasn't changed all that much.

I would balance price with target audience rating. If the target audience rating is high, then spend more on the latest MCO. If the target audience rating is low, save money on a second hand, older MCO.

This advice is based on the fact that at lower levels, opening theory isn't all that important, so paying a lot of money for an opening encyclopedia doesn't make much sense.

One final note is that openings come in-and-out of fashion. You may find that differences in MCO editions over time reflect more the level of popularity of an opening rather than any substantial changes to chess theory.