When Lester found a book of haiku in the bookstore, why did he almost put it back?

This is the way he describes about what happened to him.

I wondered. I opened the book and read,

On a withered branch

a crow has settled —

autumn nightfall.

I trembled and turned the pages hastily until my eyes stopped on these words:

A giant firefly;

that way, this way, that way, this —

and it passes by.

it is an infinitive phrase with a subject.

hey u think u can me with a summary? ?