To tell you the truth, I havn't.
But I almost bought a book about writing buisness plans a while back, but never did for some reason.
I think buisness plans are necasary for any buisness, but especially for offline buisnesses. Its much easier to run and work on an online buisness, because they're are much less legal issues and concerns as theyre might be offline. But still, having a buisness plan, or taking the time to write one, for an online buisness could help you alot, so you know where your going and have, sort of, a track your following.
So I'd suggest you go on
www.amazon.com and search for some good buisness plan books. Be sure to look at the book reviews before buying so you get a book other people thought were good.
Good luck,
Brandon