These are software systems that are often required by many startups. If I am developing a SAAS web application that may require monthly/yearly recurring billing(with various discounts/offers) and I have no prior experience of this, where do I find information about the best practices to develop such system? Ofcourse one can try to design and develop it as any other piece of software but it will make life very easy if one can learn about design/architecture of such reusable systems.

In short, I am looking for something like : The Architecture of Open Source Applications(http://www.aosabook.org/en/index.html) but for web application components like Authentication, Billing, Forums, E-commerce etc.

Or, may be like this : http://stackoverflow.com/questions/549/the-definitive-guide-to-forms-based-website-authentication