You can find the official documentation here.
Maybe you’re not yet ready to jump into SwiftUI but that doesn’t stop you from getting started with Combine if you can require iOS 13. It’s already useful on its own and you’ll have one less new thing to understand when you do decide to learn SwiftUI. It’s easy to get bogged down in the theory so I thought I’d start with a practical, but hopefully still understandable, example.
Discover how Apple’s Combine framework can be used to model increasingly complex asynchronous operations as reactive pipelines that emit values over time.
This is an intermediate to advanced book, focusing narrowly on how to use the Combine framework. The writing and examples expect that you have a solid understanding of Swift including reference and value types, protocols, and familiarity with using common elements from the Foundation framework.