According to the Wall Street Journal, voice-over actors make good money by recording political advertisements. The Boston Public Library has a number of books on careers as voice artists:
The Art of Voice Acting
The Craft and Business of Performing for Voice-over
Alburger, James R. (Book – 2011)
Secrets of Voice-over Success
Top Voice-over Actors Reveal How They Did It
Baker, Joan (Book – 2009)
Voice Actor’s Guide to Recording at Home and on the Road
Hogan, Harlan (Book – 2009)