Which you prefer ? and why ?
I bought modern chess openings, and it's fine as a reference, but everyday I wonder why I spend money on it when there are online databases, ha ha. One thing that's nice about the book is that he gives one or two page intros to each opening, describing the strategies and stuff.
it means there are no theories for opening? I mean why it is played or why this particular move is better than other moves?
NCO has better lines (I do not have it). MCO is a good reference for basic opening preparation until you encounter strong opponents that are serious about opening study. They will take you out of MCO pretty quick.
I use ECO plus databases from which I have culled much of the junk (quick draws and games between low rated players). But, I refer to MCO often enough that it's useful having it beside my desk. If I did not have ECO (both print and electronic versions), I would strongly condider getting NCO.
So which version of ECO is best? I mean they are written by lots of different authors and updated regularly.
NCO is quality but a bit old, but now with the advent of computer databases they are all a bit unnecessary.