Applications that can be written using Node.js include, but are not limited to: Static file servers, Web Application frameworks, Messaging middle-ware, Servers for HTML5 multi-player games, etc.
Node.js’ package ecosystem, npm, is the largest ecosystem of open source libraries in the world. It provides access to a massive list of modules (libraries) to minimize your coding efforts.
I recommend “Node.js in Action” and “Hands on Node.js” for people eager to learn Node.js.
For a different learning experience, I recommend the learnyounode interactive course by nodeschool.io. You can get it from here: https://github.com/workshopper/learnyounode or by running the script “npm install learnyounode” (In case you have npm installed).