It is worth it, and not only because it is a classic of Brazilian literature. Dom Casmurro tells a simple story, the childhood love of Bentinho and Capitu, the marriage and the jealousy, in 148 short chapters, and the question the reader carries to the end, whether Capitu cheated on her husband with his best friend, is answered only through the voice of the narrator, who is an interested party.
What the book gives the reader is the pleasure of reading with suspicion. The one who tells everything is Bento Santiago, now old, who warns right at the start that he will cut what he does not want to tell ("Childhood confessions, you would be worth two or three pages, but I want to be spared.") and who writes already convinced of his wife's guilt. Each scene asks that what happens be separated from what the narrator concludes, and that is where images such as the eyes of undertow ("Eyes of undertow? Well, of undertow.") gain two meanings.
What can get in the way is the pace. Much of the book is childhood and seminary, with promises, dependents and family conversations in Rio of 1857, and the suspicion only grows in the last third. The narrator also interrupts the story to speak to the reader, which amuses some and tires others.
The ending rewards patience: the narrator ties the loose ends together in the image of the girl who was already inside the woman, "as the fruit inside the peel," and moves on without further explanation ("Let us turn to the History of the Suburbs."). Whoever closes the book usually wants to go back to the beginning to check the evidence, and that is the best praise it receives.