These are some good, useful, or insightful references.
Plait Language has some good info on design decisions that make a type system in Lisp a little bit better.
Typing Haskell in Haskell The implentation of Hindley-Milner in Typing Haskell in Haskell was ported to common lisp and serves as the base of coalton's type system.
Type classes: an exploration of the design space This paper guided the typeclass implementation in Coalton.
Elm Lang Elm's runtime currying system directly inspired the model used for function application in Coalton.
Type Inference for Haskell by Jeremy Mikkola Jeremy's blog series was very helpful for understanding some of the more intricate parts of THIH.