E-commerce is literally the most rewarding part of a web application — you can see money flowing into your company due directly to the code you have written. But dealing with payments and payment gateways is complicated and stressful. It's often the most complicated and precise business logic in a system.
When I started on my first applications that truly had complex logic around payments, I was surprised at how little information there was about managing the entire ecosystem surrounding a payment. Not just the API call to Stripe or Braintree or PayPal, but how to identify the information you need to store locally, how to prevent bad outcomes from network failure and how to deal with fraud.
Eventually, I decided to write some of these things down myself, and the resulting book, Take My Money: Accepting Payments on the Web is available today as an in-progress beta book from Pragmatic Press. Inspired by work for Table XI clients, Take My Money: Accepting Payments on the Web, guides developers through the setup and complications of dealing with online financial transactions. It includes software design, securely managing payments via Stripe and PayPal, managing recurring subscription payments, administration of an e-commerce application and reporting.
The book is available today at http://www.pragprog.com/books/nrwebpay. I'm proud of it, and I hope it will help you build your web business.