Having read some books, leafed through many I have found that only very few books really helped me improve.  Over all I find good books keep coming but sorting out the good ones becomes increasingly hard as the number of lower quality books seems to be exploding.  So I thought, why not ask here and see what experiences others have had.

Which book had the greatest influence on your quality of play?  Pick your ONE title that did the trick for you.

Not a book but a massive compendium of articles => Dan Heisman's Novice Nooks on chesscafe.com

Reinfeld's Winning Chess Openings, which led me to MCO-10 a few years later.

Being fascinated with openings didn't do my game much good. So, a bad influence in terms of results but still pleasurable.

Winning Chess Strategies by Yasser Seirawan has had the greatest influence on my quality of play. But that does not necessarily mean it's a good book. It improved my play the most because before reading it I knew very little, and had read even less, so I couldn't help but improve dramatically.

WCS doesn't have any startling new chess insights, but it's well written and presented, meaning you actually do read the whole thing. If your question is code for, "What is the best chess book in your opinion?", WCS is probably not the one I'd pick.

The Amateur's Mind by Jeremy Silman gave me most of the positional understanding I have today. I'm only just starting How To Reassess Your Chess by Silman, which may be better for your level if you're also around 2100.

'Bobby Fischer Teaches Chess"

Thanks for your thoughts. My favorite:  Chess strategy for the club player, by some dutch guy. Boy did I wish I had this book 25 years ago.

 thanks for the hint I will have to check this one out, the other titles I had come accross before but this one is new to me.

I am suffering from having too many chess books and continue to buy more before completing any book at all.

After continuously buying chess books before completing the one I own, I finally found a chess book that has hooked mere more than any other book. It is Giants of Chess Strategy.

The authors explainations are so good and just by going through one game from the book each day I feel improvement in my own chess thinking.

I suffer a lot from reading 10 pages of a chess book, and the next day most of it is forgotten. This book is an exception and will without doubt improve my chess!

I just hope that his other book in the Giants of Chess series is just as good....

Nimzovitsch 'My system' hands down - no other book comes even close for me( I did read about 30 in my life). Nimzovitsch had a gift to explain even difficult positional play in simple way so even intermediate/beginner can understand it.