If we look at this poem literally, we find a librarian in a library witnessing some crazy events. First, she sees a man eating books. Then, she sees mad dogs running up the basement stairs. Lastly, the speaker gets on his knees like a dog and licks the librarian's hand. If we look at the poem in this way, it quickly becomes apparent why the librarian shifts her mood and behaviors in various ways.
The librarian "does not believe what she sees" as she watches the speaker eat the poetry books in her library. How would you react if you saw this scene? She doesn't seem to know how to react, as "as she walks with her hands in her dress," which could imply defensive body positioning. In this moment, her eyes are described as "sad." Not only is she shocked by this sight, but she is also mourning the loss of her books.
However, the confusion and sadness are replaced with rage as she sees dogs running up the basement stairs. As the dogs run, she "begins to stamp her feet and weep." She can't handle this chaos, but her childlike tantrum shifts again when the speaker, a human man, gets on his knees and licks her hand like a dog. The librarian "screams" in response.
As the reader moves through the stanzas, they see the librarian shifting from sad and confused to angry to horrified when the poem ends with a grown man licking her hand like an animal. Her behaviors shift based on the irrational scenes occurring in front of her eyes.