There are various free or relatively inexpensive lisp books out there (covering Common Lisp, Scheme, and Racket in particular). Some may debate whether lisps are functional. They're more pragmatic in some ways than the ML family, but do permit functional programming. Structure and Interpretation of Computer Programs, How to Design Programs, Paradigms of AI Programming as will as On Lisp and Practical Common Lisp are ones I can recommend (having worked through all or large portions of them). All those are free. Little Schemer and Seasoned Schemer aren't free but aren't expensive, the style isn't for everyone but I liked them.
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.
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.
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
http://learnyouahaskell.com/
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.