Nik Graf has a free ReasonML course on Egghead.io, which has a lot of positive reviews from the community. https://egghead.io/courses/get-started-with-reason
Dr. Axel Rauschmayer has written "Exploring ReasonML and functional programming" which covers the language comprehensively. http://reasonmlhub.com/exploring-reasonml/
Web Development with ReasonML by J. David Eisenberg, published by Pragmatic Bookshelfis in the works. I read a few sample chapters and they're excellent for beginners: https://pragprog.com/book/reasonml/web-development-with-reas...
Jane Street wrote a set of 24 runnable exercises that teaches OCaml, and we ported it to ReasonML. It has been used for workshops and individual study and will get you upto speed on the paradigm quickly. https://github.com/protoship/learn-reasonml-workshop
You also have textbooks on OCaml, a lot of them openly accessible, including the venerable Real World OCaml. They are all listed in the awesome-ocaml repository: https://github.com/rizo/awesome-ocaml#books
There is vramana's awesome-reasonml repository that covers similar ground for Reason itself: https://github.com/vramana/awesome-reasonml