While I have yet to come across any resources specifially targeted to accountants, I recommend Steve Hackney's work on marketing service businesses.
He has developed a very comprehensive and impressive step-by-step sales and marketing system, which can be easily tailored to the specific needs of accountants.
To get a first taste of his system, he has written a five part marketing course which can be signed up for at his website, Power Marketing Strategy.
If you have come across any other useful resources for developing accountancy businesses, such as books or websites, please let me know by contacting us.
Alternatively you can use our consulting business plan system here for setting up your accounting practice