I often use Google Books to preview books that I am interested in, such as David Campany's Photography and cinema below.
However, there is also a lot of material available in full. For example, I recently found copies of Life magazine ("the treasured photographic magazine that chronicled the 20th Century") from 1953-72 [UPDATE - NOW AVAILABLE FROM 1936].
Available here: http://books.google.co.uk/books/serial/ISSN:00243019
Issues of LIFE are now available from November 1936