Plenty of folks have written on Critical Mass and its role in transforming society. I haven't ridden in Critical Mass in too many places - Chicago in '04 and '05 (absolutely phenomenal - bikes as far as you could see), NYC in '08 (well, we didn't actually ride... because there were about 30 cops and ten cyclists in Union Square with us), Austin '09 (Austin gets more riders on the Thursday Social Ride than on Critical Mass) - but even my small trajectory reflects the decline of Critical Mass' political power over the past decade. I'm torn. Either Mass is being dismantled because it threatens to upset the dominant social order, or it's being dismantled from within because it's already served its useful purpose and gotten bikes on the map. Portland and NYC (both represented below) tell very different stories.
Either way, here are a few interesting sources on Critical Mass.
Carlsson, Chris, ed. Critical Mass: Bicycling's Defiant Celebration. Oakland, CA: AK Press, 2002: AK is an anarchist press, which is rather fitting for a whole book on an anarchist event. Though Carlsson is described elsewhere as the founder of Critical Mass, in true anarchist fashion he totally denies any association beyond collecting, compiling, and publishing the dozens of short essays that make up this volume. Critical Mass is a once-a-month spontaneous cycling event where cyclists in cities all over the world band together and ride en masse through city streets during Friday rush hour. As an anarchist event, it has no leader, founder, set route, or set meaning, and though the many Massers who contributed essays to this book all agree that it has something to do with a lot of people bicycling at the same time in the same place, very few agree as to why they are doing it. This lack of agreement makes for a fascinating volume written by people of all levels of literacy, humor, and philosophical engagement and provides a lot of food for thought as to the relationship between riding/writing and doing/speaking. (Check out some other stuff on Carlsson and Critical Mass here.)
Lynn, Andrew, Elizabeth Press, and Chris Ryan. "Still We Ride." United States: In Tandem Productions, 2005: Still We Ride is a film made by the NYC activist organization TimesUp! that documents the police crackdown on the Critical Mass ride during the 2004 Republican National Convention. It is obviously biased toward showing that cyclists are not considered “traffic,” and at times those being documented seem to be actively provoking the police, but both the bias and the obvious acts of police brutality (beating cyclists, seizing bikes, etc.) make a compelling argument for how difficult it is to be a transportation minority in a car culture. Made just six years ago, it also serves as a historical record of a car-dominant society in an increasingly multi-modal culture: just this fall, 80 of the 240 cyclists arrested that night were awarded a multi-million dollar settlement from the New York City government.
Furness, Zack. "Critical Mass, Urban Space and Velomobility." Mobilities 2, no. 2 (2007): 299-319: This article focuses on the spatial politics of Critical Mass. Using the Situationists, a 1950s group that attempted to revolutionize society by temporarily appropriating spaces and transforming them through spontaneous performative critique, he characterizes Critical Mass as a performative critique of car culture.
Sano, Rev Phil, and Joe Biel. "A Post-Critical Mass Portland: Living in a Post-Revolutionary Bicycle Age." United States: Microcosm Publishing, 2009: This short video examines the reasons why Critical Mass in Portland no longer has the revolutionary (velorutionary) power it had in the early 2000s. Its conclusion that Portland would rather spend thousands of dollars silencing Critical Mass than allow traffic to be impeded is similar to the TimesUp! argument in “Still We Ride,” but with an important caveat: unlike New York, Portland has been named the most Bicycle Friendly Community by the League of American Bicyclists for several years and boasts not only hundreds of miles of bike friendly infrastructure but a significant modal split and a thriving bike culture and bike industry. Though the filmmakers do not suggest this, the end of Critical Mass in Portland may mean a silencing of cyclists, but it may also indicate that Critical Mass has served its purpose and made its voice heard, and that it was in its death throes, anyway.