This is a book that I have yet to read, but I might want to when I get more time for it. Nimzowitsch apparently has a nice way at simplifying the chess position. For example, the topic of "tempo" in chess is one tough to explain, yet Nimzowitsch does so expertly (so I hear).
I don't know if there is one reason why this book is as renowned as it is, but every subject has its select cannon works and authors that everyone "must read." If you are into philosophy, then you will read Socrates and Aristotle and such eventually. If you are into Economics, then you will read from John Maynard Keynes eventually. In chess, I'd argue that most of the strong players eventually read My System by Aron Nimzowitsch. Another huge book for chess players is My Memorable 60 Games, by none other than Bobby Fischer himself (the chess book I am currently reading). Also, it is not a book per se, but I think nearly all strong chess players are familiar with classical players strengths and learning from them. To study endgames, perhaps Capablanca or Kramnik. To study positional ideas, perhaps Karpov or Morphy. To study tactical attacks, perhaps Alekhine or Tal. Sure the list goes on, but the general idea is not who you follow or what exact books you read, but that you focus on topics that are strengths of the author.
My System is simply one of those well-known works that just seems to come up time and time again.
Why it is considered the most important study book to have?Aron Nimzowitsch wrote My System more than 75 year ago and the book has made effect on thousand chess players all over world.