Introduction to Functional Programming (1988) [pdf]
usi-pl.github.io
usi-pl.github.io
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.
https://wiki.haskell.org/Learning_Haskell
Erlang via the link below. It's a bit different than the other two, but is definitely a functional programming language.
http://learnyousomeerlang.com/
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.
[0] https://www.cs.ox.ac.uk/publications/books/functional/
[1] https://www.safaribooksonline.com/library/view/thinking-func...