If you want to read about using Atom or developing packages in Atom, the [Atom Flight Manual](https://flight-manual.atom.io) is free and available online. You can find the source to the manual in [atom/flight-manual.atom.io](https://github.com/atom/flight-manual.atom.io).
The [API reference](https://atom.io/docs/api) for developing packages is also documented on Atom.io.