I'm most of the way through John Steinbeck's Cannery Row (published in 1945), only to discover that the novel lacks anything vaguely resembling a central plot.
I should mention in preface that I am generally a fan of Steinbeck. The Grapes of Wrath, The Winter of Our Discontent, and East of Eden are among my favorite mid-20th century novels. All contain strong plots, strong main characters, and plenty of conflict.
Cannery Row, meanwhile, is comprised of a series of vignettes that are loosely connected, all of which take place in Monterrey, California. The storyline might best be described as a series of "character studies".
Structurally, Cannery Row reminds me a bit of William Saroyan's The Human Comedy, which was published two years earlier (though Cannery Row is much, much darker and far more adult in its themes and subject matter).
Cannery Row is probably worth reading if you are a diehard Steinbeck fan, but I would recommend that the neophyte begin elsewhere. If you are new to Steinbeck, start with one of the following:
East of Eden
The Winter of Our Discontent
The Grapes of Wrath
Of Mice and Men
These five, of course, are mainstays of high school literature classes, and perhaps for good reason. Steinbeck wrote some great novels, but I wouldn't necessarily say that his novels were all great.