This Cornell gitbook is creative commons licensed and teaches both functional programming concepts and OCaml datastructures

  wget -mkEpnp http://www.cs.cornell.edu/courses/cs3110/2018fa/textbook/

Plus this talk https://www.youtube.com/watch?v=DM2hEBwEWPc Effective ML is interesting, shows how to better write ML family programs so other people can read them, and making illegal states unrepresentable where he talks through splicing up interfaces to make them more robust by taking advantage of the type system.