Secrets of Modern Chess Strategy- John Watson. A milestone in the history of chess. Maybe the most instructive chess book ever existed. It is so brillant, I would recommend it to everyone.

If u find something send to me .

modern chess strategy I II III pachman

I can recommend quite a bit of books for your level, pick and choose! :)

All of these books/series come highly recommended

How to Reassess your Chess - Jeremy Silman

Understanding Chess: Move by Move - John Nunn

Understanding Chess Middlegames - John Nunn

John Nunn's Chess Course - John Nunn 

John Nunn's Chess Puzzle Book - John Nunn

Secrets of Modern Chess Strategy - John Watson (already mentioned)

Artur Yusupov's Award Winning Training Course

School of Chess Excellence: Strategic Play - Mark Dvoretsky

School of Future Chess Champions: Secrets of Chess Training - Mark Dvoretsky

School of Future Chess Champions: Positional Play - Mark Dvoretsky

Dvoretsky's Analytical Manual - Mark Dvoretsky

Winning Chess Strategies - Yasser Seirawan 

The Most Instructive Games of Chess Ever Played - Irving Chernev

Techniques of Postional Play - Valeri Bronznik

Pump up Your Rating - Axel Smith

Chess Strategy for Club Players - Herman Grooten 

Pawn Structure Chess - Andrew Soltis

Karpov's Strategic Wins Volume 1 - Tibor Karolyi

Lasker's Manual of Chess - Emmanuel Lasker

The Mammoth Book of the World's Greatest Chess Games - Graham Burgess, John Nunn, John Emms

The Ultimate Chess Puzzle Book - John Emms

Modern Chess Strategy - Ludek Pachman (already mentioned)

Practical Chess Exercises - Ray Cheng

How to Play Chess Endgames - Karsten Muller, Wolfgang Pajeken (Will also help to improve middlegame play) 

So there it is, the cream of the crop, please do not be put off by the titles of the books as they are also rich in strategical themes and are challenging. 

As it is, the above poster has named quite few books (all very good). Only thing I could possibly add is:

New Ideas in Chess by Larry Evans

Any book by Karpov

Lev Alburt books

If this seems abit sparse, it's only because at your level, there aren't many books that will improve your play. Only referrence material or databases are helpful. Good luck.

8)

All i know is apart from a trusted good Mentor plus friend one must have alot of good books on how to stay Physically, Mentally, Spiritually(i am not sure about this one!) and Emotionally fit. Meditation also helps alot. 

A player at that level looking to improve is not looking for easy. If Nimzowitsch is difficult, that might be because it is the right book.

i guess it is, as Nimzowitch was a great player! like Tchigorin,Tarrasch,Philidor,Steinitz,Euwe and the list is endless. Given the fact that they had less technological resources at that time and that is brilliant though i am not sure as always.

I think you are right as you are a very good rating.

Re-Assess Your Chess: Workbook by Silman

His writing is very "user friendly".  He gives you a system for evaluating a position, and gives many practical examples. 

Thanks, Cdowis