To plan your writing and keep notes of ideas I suggest using org-mode.org. In the beginning it’s very easy to learn, but it grows together with your need to a full publishing machine that can help you writing your book which you can export in HTML, PDF, Latex and even to MS Word and LibreOffice (see http://vimeo.com/31564708).
To get better and practice more often tasks and writing games are a good idea such as “Give me a word and I write 15 minutes what comes to my mind” and if you dont keep up writing Emacs will automatically start deleting the last words. This is what Write or die! mode is for.
Now that you’ve written something Emacs can (to some degree) tell you how “good” e.g. complicated style your text is – like this sentence – by performing the Flesch-Kincaid scoring on it implemented by the writegood-mode
Another barometer for good writing style is the usage of weasel words als well as telling something in passive voice. artbollocks-mode can help you to avoid this (at least if you’re writing in English or are willing to modify the variables