Hi Zak
My thoughts are that unless you already have a book I wouldn't specifically go out and buy one. This is one area where I think the internet has more than enough for anybody. Google 'how to write a research proposal' or a variation of and you will get loads of info. I would stick to those on university websites as an overall guide. You will probably find that you will use a bit of this, and a bit of that.
I don't know your geo-context but you could check out a scholarship website - while you may not be applying for that particular scholarship, most of the application forms will give you a good idea of the key ideas to hit on.
Other than that any of the 'how to get a PhD' books usually have the proposal at the start. 'How to get a PhD' by Phillips and Pugh has this info I think in it (sorry, I don't have my copy with me). This (and others) should be in your university library, shelved somewhere around the '378s' - 370s in the Dewey Decimal catalogue system all relate to education in some way.