my two contenders for worst book ever are:
1) Why You Lose at Chess by Tim Harding
Harding's book is one of the least educational books I have ever read on any subject. The explanations were vapid and incomplete. And those were the good ones!
2) Openings for Amateurs by Pete Tamburro
My displeasure with this book may have more to do with the back cover blurb from the publisher.
They make a big deal about the Opening Primer section of the book and that was pretty bad. Some examples had decent explanations but most of them did not.
The "repertoire" in the back is a little better but still not very good at explaining the openings.
Honorable mention for worst booke ever is the vastly overrated snooze fest: A First Book of Morphy.
Somehow made Morphy's games boring and uneducational all at once!
Are we talking about the worst chess book published or are we talking about the least beneficial chess book for any or specific player?
Yes.