Another here that has both road and mtb versions. Both are very good imo. You have to keep in mind that they are sometimes general, and the specifics of the task you're actually doing may differ slightly, but that's not to worry, the general principles are there. Both books are excellent for just pointing you in the right direction.
I too use the internet (youtuve vids etc) as they often help you visually with the task. That said, some of the youtube clips can be overly simplistic, and assume your bike is in good nick. For example, you might have a well stuck bottom bracket, the vid clip will say something 'just remove the bottom bracket'!! NO indication of what to do if the beggar wont shift!
The advantage of books over the tinternet is that you can sit the book alongside you in the worksop (my Zinns have oily finger prints all over!). I built a wheel usiing only Zinn too.