A speaker reads aloud to a group of hogs, and the strange, small scene turns into a quiet reflection on what books are and what they do. The title, "Captive audience," is a joke at first: hogs penned in a farmyard can't walk away from anyone's reading. But the poem lets the word "captive" carry a second weight. If the listeners are confined, then the words they hear stand in sharp contrast to that confinement.
The key moment comes in the last two lines, where the books "come to taste / of liberty." Reading to animals who can't leave changes how the speaker experiences the books themselves. Words about freedom, or simply words that open up imagination, feel sharper in front of listeners who have none.
The mood is gentle, wry, and a little tender. The speaker seems amused by the odd habit ("as I have done" suggests it's a routine), yet also thoughtful. Judging by the tone, the speaker isn't mocking the hogs but finding something meaningful in sharing language with them.