Everyone I talked to who had read The Pearl disliked it. Even the normally mild-mannered Christopher called it "dreck" in recent review of Of Mice and Men. So, is The Pearl really the nadir of Steinbeck's bibliography?
I haven't read nearly everything of Steinbeck's, but out of his popular works (Of Mice and Men, East of Eden, The Grapes of Wrath, and The Pearl), the latter is easily the weakest. I wouldn't classify it as dreck, but it has some serious problems. Complete spoilers follow.
The plot concerns a poor pearl diver named Kino whose child is stung by a scorpion. While diving for a pearl big enough to pay the doctor to treat his son, he finds "the pearl of the world," a perfect pearl bigger than an egg. What initially seems like good fortune turns dark, as the villagers turn against him and try to steal the pearl, the doctor poisons the child to extract more money from Kino, and the pearl merchants attempt to rip him off by convincing him that such a large pearl is nothing more than a novelty. Kino decides to journey to the city, a couple weeks' journey, but soon after leaving, he is pursued by several men intent on killing him and taking the pearl. The chase ends in the death of Kino's baby, and he throws the pearl into the ocean, blaming it for the ruin of his life.
The biggest problem with The Pearl is the the plot doesn't progress logically. Within a day, the villagers have gone from marching to the doctor's house with Kino to demand care for his son to attempting to kill him and steal his pearl. Kino is initially a sympathetic character, but as the pearl begins to corrupt even him, he becomes less so, hitting his wife and risking the life of his family to get what he believes the pearl is worth. The book is also muddled because the actual message of the fable is unclear. Is the message that Kino was too greedy, that the townspeople were fickle, that wealth corrupts, or that pearls are evil? Is it a thinly veiled parable for the benefits of socialism (of which Steinbeck was a proponent)? Is it a cautionary tale about the dangers of individual betterment? It's hard to tell, and although this ambiguity is probably why the book is assigned reading in most high schools, it's not particularly complex or satisfying.