One avenue that people tend to ignore is to look under the hood of code you already use. I've learned an immense amount from looking at the source code of open

One avenue that people tend to ignore is to look under the hood of code you already use. I've learned an immense amount from looking at the source code of open source packages I use, and seeing how the internal code is structured, what tradeoffs they make, etc. It's not a mentor, but it can open up some options beyond books and stack overflow, especially if the projects you look at are full of comments.