:editor's hat on:
Try tracing the unknown poem. The other author had to have found it *somewhere* unless he or she made it up in the first place, in which case it's under copyright.
Determine if it IS indeed unknown. Google is your very good friend for tracking quotes. Perhaps when that other book was written search engines had yet to be invented.
But question yourself just
why it's so necessary that you use the
same poem as the other writer. Should that writer read your work and see the poem, he or she might wonder why you couldn't have found some other poem for the job.
It's one thing to drop in a quote from Shakespeare, heck, everyone does that, but to repeat a specific poem--perhaps to achieve the same effect as the other writer--just doesn't sit right.
Also question if the poem IS going to get the effect you want!
A novelist I know decided she absolutely had to have a quote from a T.S. Eliot poem. She felt it was the perfect way to begin her book. She found she had to pay a large chunk of change--perhaps a third of her advance--to the Eliot estate to use the quote.
She felt pretty good about it until she began asking readers what they thought of the quote in conjunction with the rest of the book. Nearly ALL of them had skipped the page entirely to get to the first line of her writing.
Many didn't even know she had a poem there at all. The few that did said, "Oh, it was nice" then quickly moved on to topics more interesting to them. It made no impression at all.
My friend's intent to "set the literary stage" with a cool poem fell flat, she was out of pocket for the money handed over to the estate, and she's not repeated it since. (As she's got over 60 books in print I'm thinking she knows what she's talking about.)
You might want to check google for poetry + public domain for resources to poems that are truly available for free. Also
Project Gutenberg.
:writer's hat on:
Just so you know, I'm in that number who skips reading the preface poems, quotes, etc. to get to the first real line in a book.
Unless it's Shakespeare. I've a soft spot for Will.