From everything.

I believe that books are not the only source of knowledge. It's true that: A book is the window of the world.

But hey, I guess when you build the house of knowledge, it might consist of windows, doors, and also the chimney. There might be newspapers, dictionaries, movies, musics, articles on the internet, or even by just looking to the nature.
To make the house more bold, more colorful, and more beautiful for what it stands, then you need to paint the walls. You can discuss with people override their race, religion, or political preferences.

So now, have you got what I mean?

Learn from everything. Furthermore, you will be led into the deepest of wisdom.