Martin Gardner's Scientific American columns on flexagons helped expand the audience quite a lot.
They've been reprinted along with collections of some of his other columns.
Flexagons, Oakley & Wisner, 1957.
The first paper on flexagons.
Flexagons, Conrad & Hartline, 1962.
Describes a lot of theory behind how to create and analyze flexagons of various types.
Various papers from Harold V. McIntosh.
Describes theory behind creating and analyzing flexagons created from different shapes.
V-Flexing the Hexahexaflexagon, T. Bruce McLean, 1979.
First introduction of the v-flex.
The combinatorics of all regular flexagons
by Thomas Anderson, T. Bruce McLean, Homeira Pajoohesh, ChasenSmith
The Flexagon Portal.
Elegant designs on hexaflexagons, dodecaflexagons and more.
Flexagon Newsgroup on Yahoo!
Contains discussions and announcements on a wide variety of flexagon topics.
Les Pook's blog on flexagons.
He's also written multiple books about flexagons.
Check out Flexagons Inside Out
and Serious Fun with Flexagons.