Chaucer (c1320-1400) is often regarded as the father of English literature, having written an extensive amount of English poetry, most famously The Canterbury Tales.
He lived before the invention of the printing press, and it has been hard to establish exactly which poems are his.
For centuries, many works were wrongly attributed to him.
Dr Marshall, who is a senior lecturer in English, said that when her find was confirmed she felt "amazement that something could still be discovered about Chaucer".
While visiting the Bodleian Library at Oxford University, England, in January this year and waiting for the edition to be brought to her for the first time, she had confirmed that the book - titled The Poetical Works of Geoffrey Chaucer, and published in 1807 - was not listed in main bibliographies of the period.
"I'm about to see an edition that officially doesn't exist," she recalled.
When it arrived, she saw the edition was in seven volumes, of pocket-book size, able to be easily carried by readers.
"The discovery of this edition changes the way we had thought people in the 19th century understood Chaucer's poetry.
Until now we've assumed that in this period Chaucer was of limited interest."
This edition showed that people in the 19th century were "interested in Chaucer, and made intellectual decisions about what was or was not important about his poetry, nearly 100 years before we previously thought this occurred".
"They were quite discriminating."
Previously, the earliest-known Complete Works of Chaucer that noted that some poems previously attributed to him were spurious, and some were authentic, had been published in 1897.
The 1807 edition identified the wrongly-attributed poems much earlier than had been thought, she said.
She discovered the book, referred to in a resource text on multi-volume poetry books, while researching a poem attributed to Chaucer, for her recently published book The Anonymous Text.
The Chaucer edition's existence had been noted by scholars as early as 1908, but its significance had not been identified, and the edition had not been previously studied.
