I agree with Petar Bosnjak. Each opening has an idea behind it. Get to undertstand this idea and if your opponent varies from the main line you should know how to punish him.
As for why books don´ t include every single wrong variation is obvious - it would only help against very weak players and would take up too much space. The book would be huge!
As for why books don´ t include every single wrong variation is obvious - it would only help against very weak players and would take up too much space. The book would be huge!