Derek Oppen, "Prettyprinting" in TOPLAS '80
John Hughes, "The Design of a Pretty-printing Library", in Advanced Functional Programming '95
Norman Ramsey, "Unparsing Expressions with Prefix and Postfix Operators", Software Practice and Experience '98
These three constitute a linear order as a subset of the directed citation graph. :) In another thread of work, there is
Olivier Danvy, "Functional Unparsing", BRICS tech report '98
which has a rather different meaning of "unparsing". The first three papers are about laying out code with indentation and parens and stuff — finding a one-sided inverse to parsing — whereas the Danvy paper is about coming up with a little language of combinators to simulate printf in a civilized way, suitable for languages that decline to admit such barbarisms with scarce questions and open arms.