Node.js is just JavaScript running on the server side. That's it. That's all there is to it.
- Express Docs, if you want to get started and already know JavaScript this is the place to be
- JavaScript For Cats
- A Re-Introduction to JavaScript
- JavaScript Reference
- JavaScript Guide
- Why Use Closure
- Prototypal Inheritance
- Control Flow in Node
- Truthy and Falsey Values
- How to Write Asynchronous Code
I think, Node.js & Co. should also be mentioned. It's a (and besides that also the first) German book about Node.js and its ecosystem.