A reference of AppStream's public API can be fount on the API reference pages. AppStream provides a second library with building blocks to compose metadata catalogs. It is used by the appstreamcli compose utility as well as AppStream Generator. You can find its API description on the libappstream-compose API reference. Please keep in mind that the compose library interface is still not fully stabilized. If you want to use the PackageKit API, look at the PackageKit reference manual.