Commentary by cpressey on Formal Language works
Introduction to Formal Languages
A decent older textbook on formal languages.
Natural Language Processing Techniques in Prolog
Some introduction to DCGs and chart parsing and feature structures.
Programs, Grammars, Arguments
The Hardest Context-Free Language
It's CFL-complete, for any reasonable conception of "CFL-complete".
Lecture 7: Definite Clause Grammars
On the Structure of Context-Sensitive Grammars
Interesting technical report on (and this is the author's phrasing, not mine) "what context gets you".
It's like a Turing machine, but in some ways worse. It doesn't just have to launch the tape head flying down the tape until it reaches some area of interest (a global variable, say); it actually has to launch symbols across the input string, sliding them along like "messages", until they interact with whatever we wanted them to interact with.
In this way, it's also a lot like a 1D cellular automaton.
Functional Unification Grammar
Definite Clause Grammars for Language Analysis
Formal Languages and Infinite Groups
An exposition of formal language theory from the perspective of group theory, giving an algebraic formulation of the usual classes of formal languages.
Formal languages and groups as memory
Explores an interesting twist on "counter automata" where the "counter" is not an integer, but only an algebraic structure with (some of) the properties of an integer: a group or a monoid.