If you’ll be using graphql you might want to check out Ben’s book: Craft GraphQL APIs in Elixir with Absinthe (Pragprog) 
It’s actually a topic I’ve seen crop up a few times, and so if you aren’t intending on using graphql, it might be worth posting here: Your ideas for Elixir book authors and content creators (or, if you want to get PragProg’s attention, we have a similar thread on Devtalk in a section where one of the PragProg editors is active (we actually created the section after a conversation with her).
Other than that it might be worth checking out our API tag as we’ve had quite a few threads relating to the topic 
api
