This really depends on the type of e-commerce platform and the platform on which it is built.
For my e-commerce clients, with less than 1,000 products, the best business plan reference book would be The Success Principles by Jack Canfield. However, you must first understand the triggers you would need to captivate and persuade your audience to purchase. For that, I would recommend Fascinate by Sally Hogshead.
When you start to get into the larger e-commerce retail websites, you run into more dynamic data and database strategies to ensure that your website runs smoothly and can be indexed by the search engines properly. The book recommendation would depend on what platform your website is built. However, Don't Make Me Think by Steve Krug would be a good starting point as the business plan would be contingent on the flow of the website and customer purchase power.
If I can further help you in any way, contact me. I'm always here to help.
In my opinion no one source can give you everything you'll need to make this business plan.
Plus E-Commerce is quite a general topic. Let us know what your business sells specifically if you'd like some really great detailed reports about your market over the past 5 years, and insights for the near future.
That being said, a good business plan should cover all of the concerns of your current business model, as well as a lot of considerations for the future that you'll need to research in different locations around the web.
