Reading Nimzovich's My System in 1998 was a setback for me. It led me to think I could be like Nimzovich and play strange moves and invent my own style of chess. What brought me success later was a mindset of conformity--conformity to sound principles and standard patterns of development and movement.
A better study material was Yasser Seirawan's Pro Chess Video Set. He gave me a sense of confidence and the ability to formulate strong plans. Unfortunately, I don't know where to get those videos now.
When it comes to chess books, there is a glut of opening books and too few about endgames. You should focus on endgames if you want to improve. Grandmasters are really at fault for writing too few endgame books.
Jeremy Silman's Complete Endgame Course is a good book. Silman is an excellent writer. He tailors the material according to playing strength so just flip from the beginning until you are mystified by something and read forward from there.
Glenn Flear's Practical Endgame Play: Beyond the Basics is not as good. Flear does not give enough explanations. I was left wondering what I had paid for.
Update (2011):
Thank you alex walsh for your suggestions. I have Dvoretsky's Endgame Manual on CD ROM and Nunn's Secrets of Rook Endings and they are both excellent. I also recently bought Nunn's Chess Endings Vol 1 and 2.
Looking back at what I said about Flear's book, I was too harsh. Actually I was excited by Flear's concept of NQEs because I have wanted coverage of that subject for a long time. In fact, Flear's book makes a sort of Vol 3 to Nunn's recent series. The books are the same dimensions so they sit nicely on my shelf together.