I recently went half way in a project using Erlang back-end but didn't get to a production stage mainly due to political reasons in my organization. Mnesia (Erlang’s distributed database) is remarkable and in the Erlang way it comes completely packaged with the system. My only gripe is that documentation overall is a bit hard once you start digging in. The OTP book mentioned is good but may be a bit dated (but we can figure out what is amiss still). I am not sure whether drivers for accessing external databases such as Mongo, Postgresql etc are actively maintained. (Edit: when I check it now it looks like the drivers are recently updated!)