From the Language Design centric point-of-view, of the top of my mind. (With all those sources, the reader will have to do almost all the work themselves, but still, this is good stuff!)
•
Cognitive Grammar by Ronald Langacker (far too wordy introduction and still far away from readily applicable stuff, but this is as close as current linguistics get to practical PLD usefulness imo)
• For the language designer, learning the Language Esperanto fluently imo offers an intuitive feel/exploration tool for human grammar that not much else will
• All of
Rich Hickeys talks/transcripts will reduce fear of transforming deep philosophy into practical language design
•
Constructing the User Interface with Statecharts (yes, there is a free pdf somewhere on the web!) this is a mind-opener in a very practical sense
• David Harel’s original statechart paper and especially his paper
On Visual Formalisms (avoid all papers about how to merge statecharts with OO, as that added some tooling for handling complexity in OO, but detracted from imo more fruitful directions such as generalizing (like letting them mingle with ADT’s 🙂 ) and in general adapting statecharts for PLD needs)
I quite like historical papers and books, not so much for what they say about the future but for what they convey about the mindset of invention, here are some:
+ Andrew Hodges
Alan Turing biography (the book; avoid the film at all costs)
+ Rich Hickeys
HOPL paper
+ David Harel’s
Statecharts in the Making: A Personal Account
etc.