Hi,
There is no open source or commercial tools to actually design APIs (I should say: no tool that meet my expectations :-) and providing all the features needed to do what I describe in my book). But there are open source tools that you can use to describe your APIs taking advantage of the OpenAPI Specification. If you're into learning the OpenAPI Spec and writing YAML, you can use the good old Swagger UI (
https://github.com/swagger-api/swagger-ui). There is also Stoplight Studio (
https://github.com/stoplightio/studio) which let you describe your APIs (generating OpenAPI code under the hood) without having to write a single line of YAML.
Arnaud