I'm not qualified to give advice to anyone on business matters, so I won't. However, I will say that your current approach is opposite of what the current popular mindset is in building web startups. (Whether that mindset is actually correct in any way or not could be a matter of some debate.)
You almost certainly are going to change your project's direction over time. Things that seem do-able now, or seem to be good ideas now, will be less feasible or less shrewd six months or a year now. So, if you lock yourself into this formal and structured plan, you might be foregoing the ability to maneuver quickly and change direction later on; and if you don't give those things up, then you're wasting some resources on something that you might be throwing away later.
In terms of marketing, your best bet is to advertise what you actually have, not what you intend to build, so again, the business plan doesn't help there.
That said, I can't disagree with having some direction, some goals, a roadmap, and a well-defined project to start with. But, those things don't need to be "formal"; a whiteboard (if everyone is local), or good project management software would accomplish that just as well.
Now that I've done my utmost to avoid answering your question: one of the few business development books that I actually found helpful before launching my own business was "The Six Week Startup" [1]. It does a nice job of breaking down the entire business launching process into digestible, well-defined bits. If you really want to go with the traditional business development strategy, then I think this book might be what you're looking for.
[1]: http://www.amazon.com/Six-Week-Start-Up-Step-Step-Achieving/...