In both the first and last stanzas, Mark Strand uses the metaphor of being a dog to describe his love of poetry. A metaphor is a comparison that does not use the words like or as.
Strand's speaker likens his love of poetry to a dog's love of eating food. In the first stanza he says he has ink running from his mouth, as a dog might have bits of food or salvia running from its mouth from eating beloved food. The speaker is suggesting that he doesn't simply read poetry, he consumes it. It gives him nourishment, just as food gives a dog nourishment. Just like a dog's relationship to food, the speaker's relationship to poetry is uncomplicated: it quite simply gives him "happiness."
In the last stanza, the speaker repeats the same idea of being like a dog and being happy. He is happy because he has had the chance to spend time in a library and read—or consume—poetry. Like a dog, he "snarls" and "barks," and he also "romps" with happiness. The librarian, in contrast to him, doesn't understand the simple, primal pleasure being in a library with books brings to him. She "weeps" and "screams" as the speaker expresses his enthusiasm in what, to her, are inappropriate ways.
Overall, the speaker wants us to feel how much he loves poetry and the extent to which it makes him happy.