Library applications will usually have modules named appname _something, and one module named appname . This will usually be the interface module that’s central to the library and contains a quick way into most of the functionality provided.
By looking at the source of the module, you can figure out how it works with little effort: If the module adheres to any given behaviour (gen_server, gen_fsm, etc.), you’re most likely expected to start a process under one of your own supervisors and call it that way.
If no behaviour is included, then you probably have a functional, stateless library on your hands. For this case, the module’s exported functions should give you a quick way to understand its purpose.