One highly recommended reference book for developing a solid business plan for an e-commerce retail sales site is "The E-Myth Revisited: Why Most Small Businesses Don't Work and What to Do About It" by Michael E. Gerber. While not exclusively focused on e-commerce, it provides valuable insights into building a successful business from the ground up, including the importance of having a clear plan and strategy. Additionally, resources like "The Lean Startup" by Eric Ries and "Business Model Generation" by Alexander Osterwalder and Yves Pigneur can offer valuable frameworks and methodologies for developing your e-commerce business plan.