Generally I'd agree with the just-dive-in advice. But I did find two books particularly useful.
Firstly "Authoring a PhD" by Dunleavy. This helped me shape the overall structure of my PhD thesis, and made writing up much more tackleable than it would have been otherwise.
And, in the very final stages, "The Doctoral Examination Process" by Tinkler & Jackson. This demystified the viva, removing the terror, and helped me prepare for it in the best possible way.