I use emacsemacs, editing LaTeX files, with a text editor I am used to (as in 20+ years "used to"). This has some advantages and some disadvantages. It allows me to separate text in whatever "file chunks" I find convenient (usually chapters) and allows me to re-arrange these in the document as a whole (by having a series of include statements, referring to the chapter).
It allows me to generate decently-typeset output in the end. I also have some conversion tools that allows me to generate other formats. However, it does not fit well with any publisher's work-stream and this is a potential problem for the future.
If you don't already have a substantial amount of time invested in a specific tool, I have heard good things about Scrivener (as in, it allows you to do the things I find to be a "win" with LaTeX+text editor, with "integrates better with publishers" as a big win).