Cheryl Harris, Alison Alexander (eds). Theorizing Fandom: Fans, Subculture, and Identity. Cresskill, N.J.: Hampton Press, 1998.
This is a rather useful edited collection about various media fandoms and fan practices. There is the usual stuff on slash included. Of course the book’s appearance in 1998 means that it was published before the real take off of online fandom.
I particularly enjoyed (non slash focused) chapter 10 by Cinda Gillalan on the TV series War of the Worlds fandom and its rejection of the hero codified by the makers of the series as being attractive to female viewers in favour of a character in the series with more marginal attributes.