
Both Sinclairs paid homage to Jack London and as a result both became friends with London. The details of those initial calls to "Wolf" are in James L. Haley's biography of London.
A Tribute To Two Sinclairs - Sinclair Lewis & Upton Sinclair
Greg Mitchell's fascinating read about the connection between Upton Sinclair and modern "dirty" politics. Huffington Post discusses the book here.