Umm. 'My system' is worth reading but only so you have an understanding of what you are unlearning as you get stronger. In this day and age it is a long way from all you need to know, chess is much less dogmatic and more objective. The demands of the position trumps nimzowitsch's rules.
Everything you need is in that book. It's not too long, and very readable. It has a very common-sense approach. Look for the 21st century edition at used book stores or your chess club: http://www.amazon.com/My-System-21st-Century-Edition/dp/1880...
chesstempo.com is good for practicing tactics between games.