By line 16, we get that a transformation has indeed occurred: "I am a new man."
But we're also given some assurance that the speaker is still a "man" even if he's barking, licking, and snarling at the librarian.
And it doesn't look like the speaker is too bothered by this sudden transformation, either. In fact, he seems right at home with his snarling and barking, as if all the doggy behavior is now part of his nature.
If we think about personal experiences in a more general sense, we understand what Strand is getting at here with all the transforming weirdness that we've seen.
For the person experiencing these things, those changes inevitably become part of who he is. But for those outside of the experience, those changes often don't make sense and can be a bit frightening even.
Still, since the speaker seems to feel right at home with these changes, he doesn't want anyone spoiling it for him, so he "snarl[s]" and "bark[s]."
I romp with joy in the bookish dark.
After all is said and done, the poem has a happy ending with the speaker romping with joy in the "bookish dark."
Notice the couplet here. Strand ends his poem with the end rhyme of "bark" and "dark." That perfect rhyme really annunciates the joy the speaker is feeling, despite all the barking and "dark."
By the very end, we see even more this mingling of "joy" with the "dark" in a way that suggests that the two get along just fine.
In fact, all the burning dogs and darkness that we saw earlier seem to be less scary by line 18. The dog-man now "romp[s]" around like a puppy at a park.
Here, too, the dark is "bookish," which gives it an intellectual flair, rather than the hellish one we saw earlier with the basement and burning dogs.
By the end of the poem, all of the transitions between joy, darkness, and burning dogs seem to suggest that it is part of the speaker's delightful activity of "eating poetry." Although outside folks like the librarian just don't get it, our speaker is "a new man" because of his steady diet of poetry—deliciously transformative.