John Gutmann : the photographer at work : [exposición] Madrid, Fundación MAPFRE, 6
octubre-16 enero 2011 / [prologo Douglas R. Nickel ; texto Sally Stein], cop. 2010
LCNA : Nickel, Douglas R. (Douglas Robert) - http://authorities.loc.gov (2007-01-15)
Snapshots : the photography of everyday life, 1888 to the present / Douglas R. Nickel,
1998