Who?

My mother, whenever possible, reuses envelopes: she and I have been known to send the same battered packet back and forth across the ocean to one another until our respective post offices will no longer accept the condition, and we are obliged to locate a fresh envelope.
My uncle got a parcel in the mail this week, and he telephoned my mother the day after with a single burning question: did she, perchance, send to him a book?
She did, she said.
He had spent a long while, he said, turning the package upside down and inside out, shaking it furiously in case a note was lodged in a corner. But there was no note, not a whisper of a word. There was no return address and the writing, being in brisk capital letters, was unfamiliar and impossible to analyse.
My uncle thought and thought about who might be most likely to send a mysterious parcel. It troubled him a great deal. He wished to settle the matter and thank the sender; but there was, of course, not a shred of evidence to hang a suspicion upon, had he such an inkling to begin with.
At last, unable to either fathom or forget it, my uncle leaned forward and began, slowly, to scrape the address label from the parcel. Off went his own name and address, and concealed under it, that of my brother- a most convenient and fortunate clue that led him straight to the culprit.