The honest answer to the question "How can one learn to build API gateways?" is unfortunately "Go and build some API gateways and reflect on what you learned at

The honest answer to the question "How can one learn to build API gateways?" is unfortunately "Go and build some API gateways and reflect on what you learned at each step perhaps teaching others as you go". This isn't specific to API gateways - it's the way to learn anything effectively. Your use cases, edge cases, opportunities and pitfalls will be different from mine. Write them up, share them online (including here), so others more like you can learn when doing paper research, much like the research you've done now. Realise that next time you might need to do it differently. Name the reasons why those needs differ. Write that up. Share. It's still a youngish field that we're still trying to get right. If the existing books and blogs aren't doing it for you, it's time to figure out why and contribute back.