The documentation is all maintained in Markdown and built with Jekyll, so it's pretty straightforward. There were a few custom Jekyll tags. For writing, I used the venerable Mac editor BBEdit; obviously any text editor can do a fine job here, but there are some subtleties in BBEdit I liked. Its "open file by name" command can open multiple files with the same name at once, for instance, and its find/replace functionality goes above and beyond the call of duty.

For books/guides, it's hard to say. Find documentation you like a lot and think about what makes it good--the organization/taxonomy is really important to pay attention to, as well as the tone (formal, conversational, weird, etc.). As shocking as this might be around here, the _Microsoft Manual of Style_ is useful as a specifically technical guide, and it's good to have a relative recent edition of the _Chicago Manual of Style_ kicking around as a reference. And, just being familiar with standard grammar and punctuation rules is important. A lot of people aren't. (A lot of people who aren't still think they are.)