If it's real, it's the literary find of the century. New York antiquarian booksellers Daniel Wechsler and George Koppelman believe they have found William Shakespeare's annotated dictionary.

The book itself is John Baret's An Alvearie or Quadruple Dictionarie, published in 1580. It was listed on eBay in late April 2008. They placed a bid of $US4300 and got it for $US4050. Wechsler is unequivocal, "only $250 separated us from never having had this experience."

Although unsigned, it contains thousands of annotations in a contemporary hand that point directly to the composition of some of Shakespeare's best known works, including Hamlet, Romeo and Juliet, and many of the sonnets. Wechsler and Koppelman have spent the past six years making sense of the annotations and building a case that it is Shakespeare's copy.

To cite but one phrase attributed to Shakespeare, which appears in Hamlet:

Gertrude: Your bedded haire, like life in excrements, Start up, and stand an end.

This can be found in the dictionary under Stare: His haire Stareth or standeth on end.

There are subtle clues such as the eight examples where it is claimed he practised the letters W and S. There isn't one smoking gun, rather their case rests on the sheer accumulation of examples that Shakespeare could only have found in Baret.

From the beginning, they reached out to scholars. Wechsler explains: "They were extremely helpful giving advice, but it was also clear that they weren't about to jeopardise their reputations with such a claim."

Shakespeare biographer and scholar Stephen Greenblatt is enthusiastic about the dictionary as an unheralded Shakespeare source book. "It would reinforce, in a fascinating way, Shakespeare's passion for language. We know that Shakespeare had an eye out for unusual words – but we have only limited knowledge of where he went to find them," he said. However, he'd "not had time to weigh the evidence" of it being Shakespeare's copy.

Wechsler is prepared for the fact that no matter how strong the evidence, some people simply won't believe them. As such, they've just published Shakespeare's Beehive, a 300-page book outlining their case, which proves, at least, that the Alvearie was vital to the composition of many of Shakespeare's plays and poems. And at most, it shows that this is one of the most significant finds in the history of literature.

He feels that by opening up the dictionary to scholars, it will only reveal further evidence. "If George and I can see this, what will they find?"

Dan DeSimone at the Folger Shakespeare Library in Washington DC, said they would release an official statement on Monday.

Wechsler is reluctant to discuss numbers, but "after scholars have had time to digest the possibility and go over the evidence" they are looking to sell it. To put it in context, the last First Folio to sell at auction made £2,500,000 in London in 2006.

Eminent New York bookseller Jim Cummins, who has read the book and found the argument persuasive, said that it was impossible to guess. "Tens of millions. Someone I know thought even a hundred," he said.

At present, the dictionary is kept in a secure storage facility in New York. However, it has been digitised and can be seen online at www.shakespearesbeehive.com.