My mentor was Frank Deutschmann, who was a developer (and eventually my manager) at the second company I worked.
I am going to go out on a limb and disagree with everyone else. I think it is incredibly valuable to have a mentor. Learning from books and lots of people is simply not the same thing. You need someone who can look at what you are doing and suggest what to change. Sure, you can get lots of good advice from lots of places. But there is simply no replacement for customized feedback that is tailored for what you, specifically, are (or aren't) doing.
Now a mentor does not replace all of those other great resources. You still need them. The relationship between a mentor and resources should be an "and", not an "or".
For instance the first concrete piece of advice that I got from mine was to read Code Complete cover to cover. As he said, "It is a tome, but you need to know everything in it and internalize it." He was right. I did need to master that book, and it would be a waste of his time to teach it to me when it was right there. But after I demonstrated willingness to learn by doing so, he was willing to point me to other things, get into useful conversations, and so on. Which was incredibly valuable for me.