A letter that Deasy writes on foot and mouth disease, and gives to Stephen in 2 Nestor to deliver to the press (because he thinks his writing is a breakthrough and that Stephen is well connected in the publishing industry).
In 3 Proteus Stephen tears a corner off of it to write a poem. Of course, this mirrors Bloom tearing a corner off of his Titbits paper to wipe himself on the toilet.
In 7 Aeolus Stephen finally delivers the letter. He is embarassed to be the courier of Deasy, but has fufilled his promise. (Bloom also fulfills a promise in 6 Hades…)