Interactive ReasonML sketchbook
Effective ML by Yaron Minsky [OCaml]
Making Impossible States Impossible by Richard Feldman [Elm]
Read Designing with Modules from the Real World OCaml book (2nd edition)
A series on desigining with types in F# by Scott Wlaschin.
Refactoring UI by Steve Schoger
Design for Hackers book
Gestalt principles
The Magic Tricks of Testing by Sandi Metz (Ruby)
Don't Write Tests by John Hughes (Erlang)
Ideology by Gary Bernhardt
OCaml for the Masses by Yaron Minsky (2011)
The Design of a Pretty-printing Library by John Hughes (1995)
Out of the Tar Pit (essential vs accidental complexity)