I highly recommend novelWriter, you can click here to get a copy for Osx, Windows or Linux.

novelWriter is a simple, multi-document plain text editor using a modified markdown syntax to apply simple formatting to the text. It is designed for writing novels, and allows for the component documents to be ordered freely to create the desired structure of the novel. More details about how projects are structured are covered in Novel Structure.

In addition, the project can contain notes on the various plot elements, characters, locations, etc, that make up the story. These notes are organized in a set of category-specific top-level folders (root folders), and each entry can be tagged and cross-referenced from within the novel documents and notes. These tags make it possible to inter-link documents and generate an overview of the entire novel project and how the various documents and plot elements are interconnected. This is covered in Novel Projects and Project Notes.