Resources on reverse engineering web-applications?

I've been trying to dig into reverse engineering web-applications, I thought it would be fun to write my own desktop applications (using Go currently) that would consume web APIs (HTTP and possibly WebSocket) of some websites I use.

However, I've found this pretty hard to do, relying on Firefox's Inspector to capture requests and trying to figure out how they work - and it often gets complicated and I get cornered with some weird mechanism I can't figure out.

I was wondering if anyone has any resources (books/tools/hints) for this, and if they'd be willing to share them with me.

Thanks!