Combine, a functional reactive library from Apple, was released in beta in June 2019, alongside SwiftUI. This book explains the core concepts, provides examples and sample code, and provides a reference to the variety of tools that Combine makes available under its umbrella.
The book is a work in progress and primarily a labor of love. It is also available in HTML format online at no charge at http://heckj.github.io/swiftui-notes/. Purchasing the no-DRM epub or PDF supports continued authoring, and hiring assistance for technical & copy editing, and the creation of diagrams to continue to build on this work.