The Yellow Book
Symbolism, Imagery, Allegory
No, this isn't a reference to the now basically obsolete Yellow Pages. Dorian Gray isn't looking for the phone number of a local Domino's circa 1992.
Instead, this is a thinly veiled reference to J.K. Huysmans' À Rebours ("Against Nature"), an incredibly important novel of the Decadent period. In both the original text and Wilde's summary of it, its incredibly wealthy protagonist devotes his life to seeking as many aesthetic sensations as he can, regardless of what society says.
This protagonist is a representation of what Dorian could become—a robotic being with no true emotions and no true relationships—looking for only the next new sensation:
One hardly knew at times whether one was reading the spiritual ecstasies of some mediaeval saint or the morbid confessions of a modern sinner. It was a poisonous book. The heavy odour of incense seemed to cling about its pages and to trouble the brain. The mere cadence of the sentences, the subtle monotony of their music, so full as it was of complex refrains and movements elaborately repeated, produced in the mind of the lad, as he passed from chapter to chapter, a form of reverie, a malady of dreaming, that made him unconscious of the falling day and creeping shadows. (10.21)
Upon reading it, Dorian sees aspects of his own life reflected back at him in this character's life. However, Wilde made some notable changes (like the explicit mention of the protagonist's lost beauty, which just makes Dorian even more scared that he'll lose his looks) to make it more fitting to his novel.
Most importantly, the yellow book represents the "poisonous" influence Lord Henry has on Dorian; Henry gives the book to Dorian as a kind of experiment, and it works horrifyingly well:
There was a horrible fascination in them all. He saw them at night, and they troubled his imagination in the day. The Renaissance knew of strange manners of poisoning -- poisoning by a helmet and a lighted torch, by an embroidered glove and a jewelled fan, by a gilded pomander and by an amber chain. Dorian Gray had been poisoned by a book. There were moments when he looked on evil simply as a mode through which he could realize his conception of the beautiful. (11.37)
Its hedonistic, decadent message makes it a kind of guide book for Dorian, who lives his whole life in pursuit of its ideals.
Ultimately, as we're reminded, it's Lord Henry's fault for poisoning Dorian with the book, which comes to stand in for all of Henry's extravagant, selfish, dangerously seductive philosophical ideas.