Busking was included as alternative employment by the Princeton Review! Their book "120 Jobs that Won't Chain You to Your Desk" (published April 2007 by Random House) has a chapter about being a busker and street performer.

It includes the following sub-titles:
A relatively typical day in the life
An Extreme day in the life (featuring NYC subway musician 'Saw Lady')
Prepare for success
Getting your foot in the door
Biggest Challenges
Best perks

Busker Central is listed in the "for more info" section.