Here are a couple of secondary works plus a few books by former WUO members who aren't Bill Ayers that might be worth checking into for references to the origins of the symbol. To be fair I should mention that while I will link to Worldcat records to help folks find these in libraries, the "people who bought this also bought" feature of that big online bookseller's site was very helpful here - sad to say they do a better job than OCLC with providing a flexible, user-friendly, data rich book search. Life under capitalism.

Memoirs/primary sources:

• Sing a Battle Song: The Revolutionary Poetry, Statements, and Communiqués of the Weather Underground, 1970-1974 (annotated compilation, only partly Bill Ayers...)
• Flying Close to the Sun: My Life and Times as a Weatherman - Cathy Wilkerson
• Underground: My Life with SDS and the Weatermen - Mark Rudd
• Love and Struggle:My life in SDS, the Weather Underground, and Beyond - David Gilbert - this one's new (as of early 2012), not (yet, one hopes) widely held in libraries
• SDS/WUO, Students For A Democratic Society And The Weather Underground Organization - David Gilbert's earlier book
• For some real digging, you could look at the FBI file on the Students for a Democratic Society and the Weatherman Underground Organization - available on microfilm in several locations. (The finding aid is apparently digitized for in house use as some libraries. The record is unclear on where, exactly.)

A couple of secondary sources:

• Outlaws of America: the Weather Underground and the Politics of Solidarity - Dan Berger
• The Way the Wind Blew - Ron Jacobs