There should be no difference unless they are different updates to the item prices inside
"In all my years as a soldier, I have never seen men fight so hard." - SS Obergruppenfuhrer Wilhelm Bittrich - Arnhem
I believe that Nick hit the nail on the head. They are most likely the same book but different year version. I believe this book gets up dated and reprinted at regular intervals. Look for the printing date and get the newest version.
They are different price guide books from the same author, Detlev Niemann.
The third edition of Niemann's price guide (published in 2008) is the most recent one.
The contents were expanded and updated with every edition: The second edition (published in 2004) had 820 pages and 2421 photos; the third edition had 980 pages and 3052 photos. (Sorry, I have no data on the first edition.)
The book seen on the right is the third edition of that price guide; you can tell from the huge red "3" on the cover. This is one massive, large-sized hardcover volume. (I own a copy of the second edition.)
The one on the left is the "condensed" paperback version of the price guide containing 610 photographs on 384 pages, but covering 3,600 individual items.