Haskell Programming From First Principle

Squiggle is written in ReasonML, which is compiled to OCaml. In general, functional programming seems to be a good fit for intense language design or probabilistic implementation. There aren't many good books on the theory behind OCaml, but Haskell is similar enough (and also independently interesting).

I found this book somewhat annoying to read, but it was much more comprehensive than the alternative "introduction" Haskell books.

I also found Learn You a Haskell for Great Good to be nice but quite light, and Get Programming with Haskell to be fine enough.