That’s easy to answer. Hands down, it was when I bought the software Scrivener.
Scrivener is a word processing tool but it’s also SO handy for putting manuscripts together. You can have parts, chapters, scenes within chapters; you can use full-screen mode to stop distractions; you can edit the formatting of your writing… There are loads of features!
Not only that, but you can export your document (they call it ‘compiling’) in variety of formats, including ePub and Kindle’s .mobi file (via a plug-in). You can also export to .pdf if you want to give a digital draft to someone without them being able to edit it.
The best part? It was pretty cheap. When I bought it, I spent less than £30. It’s already paid for itself—and more.