There is no happiness like mine. I have been eating poetry. (2-3)
The cause for all the happiness is weird, but hey, happiness in general can be a weird thing too. It's unpredictable and constantly in a state of flux in both "Eating Poetry" and in life in general.
I am a new man. (16)
All the poetry eating is not only totally delightful for the speaker, but it also transforms him into "a new man." Happiness in this case can also be a transformative emotion—despite, you know, all the burning dogs and weeping librarians.
I romp with joy in the bookish dark. (18)
It looks like joy can be found in the dark too, so it's not always about cupcakes and fluffy puppies. It's a mysterious, fluctuating, and unpredictable emotion that's more complicated than it may initially appear to be.