Gender-Neutral Pronoun Frequently Asked Questions A comprehensive discussion of gender-neutral/gender-free pronouns in English over the centuries, such as 'sie', 'hir', 'ey', 'zie', singular their, and many others. http://aetherlumina.com/gnp/
Some Notes on Gender-Neutral Language Carolyn Jacobson, a Ph. D. candidate who studies Victorian English literature, provides this handout about the history of pronoun usage. http://www.english.upenn.edu/~cjacobso/gender.html
The History and Status of Singular Their (with Examples) A discussion of the traditional and established use of "they/their" as indefinite genderless pronouns by Jane Austen and other well-known authors. http://www.crossmyt.com/hc/linghebr/austheir.html