I found the 70s easy enough. It was after that it got tough to select a book that I had read that belonged on the same list as my choices for previous decades.

00s The Jungle Upton Sinclair
10s The Magnificent Ambersons Booth Tarkington
20s The Trial Franz Kafka
30s The Grapes of Wrath John Steinbeck
40s The Makioka Sisters Junchiro Tanizaki
50s The Crucible Arthur Miller
60s The Last Temptation of Christ Nikos Kazantzakis
70s The Complete Stories Flannery O'Conner
80s A Confederacy of Dunces John Kennedy Toole
80s Love in the Time of Cholera Gabriel Garcí*a Márquez
90s The Windup Bird Chronicle Haruki Murakami

Some decades the making one choice among a number outstanding books was difficult. Others it was just selecting what I considered the best of what I had read from that decade even if I would not have rated it as great.