The idea of "eating poetry" might make us chuckle a little, or think that our poet is really crazy about poetry (or, you know, just plain crazy). Whichever way we read it, the surreal imagery announced in the title tends to stick with us right from the get-go. We know things are going to get weird, but we also get the sense that there's more going on here than just a dog-man cramming poems in his mouth.
Lines 1-3: The first stanza paints the image precisely with a matter-of-fact tone that tells us this is really happening. The speaker is thoroughly enjoying himself, and there's no need for too much explanation.
Lines 7-8: It's like reaching the bottom of a Ben & Jerry's container. The "poems are gone" and the lights are dim so that joyous moment is beginning to wane. So, we get the sense that "eating poetry" is just like anything else we enjoy. The moment can't last forever.
Line 18: By the end though, the speaker "romps with joy in the bookish dark." So even though things got a little dark for a moment, the joy that comes with all that poetry eating is what we're left with (as is the speaker).