If you want an ML, there are various free resources for Haskell. The update to the linked book is mentioned in another comment but is not free. The ML family offers a different perspective on FP, particularly by way of their type system. I worked through a chunk of Learn You a Haskell, but I'm not sure how current it is. A Haskeller could probably give more feedback.
Erlang via the link below. It's a bit different than the other two, but is definitely a functional programming language.
There have been courses on Coursera and similar sites in the past that may be worth visiting, I don't know what to recommend of the presently available ones.