This link from Hacker News' frontpage catched my attention. Despite the clickbaity title, it is a thorough blog post about documentation, its role and how to write documentation users will actually enjoy reading.
To summarize, Daniele (the author of the blog post) defines four kinds of documentation: tutorials, how-to guides, explanations and reference documentation. They also explain the scope and the role of each kind, and do/don't rules for writing an effective tutorial, how-to guide, et cetera.
I am not going to summarize that great article: just go read it. Recommended!