A set of the first four editions of William Shakespeare’s collected works is expected to sell for up to 4.5 million pounds ($6 million) at auction next month.

Sotheby’s auction house announced the sale on Wednesday, Shakespeare's 461st birthday. It said the May 23 sale will be the first time since 1989 that a set of the First, Second, Third and Fourth Folios has been offered at auction as a single lot.

The auction house estimated the sale price at between 3.5 million and 4.5 million pounds.

After Shakespeare’s death in 1616, his plays were collected into a single volume by his friends John Heminges and Henry Condell, actors and shareholders in the playwright’s troupe, the King’s Men.

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The First Folio — fully titled “Mr. William Shakespeares Comedies, Histories & Tragedies” — contained 36 plays, of which half were published there for the first time. Without the book, scholars say, plays including “Macbeth,” “The Tempest” and “Twelfth Night” might have been lost. Sotheby’s called the volume “without question the most significant publication in the history of English literature.”

About 750 copies were printed in 1623, of which about 230 are known to survive. All but a few are in museums, universities or libraries. One of the few First Folios in private hands sold for $9.9 million at an auction in 2020.

The First Folio proved successful enough that a an updated edition, the Second Folio, was published in 1632, a third in 1663 and a fourth in 1685.

Although the First Folio is regarded as the most valuable, the third is the rarest, with 182 copies known to survive. It is believed the third book’s rarity is because some of the stock was destroyed in the Great Fire of London in 1666.

The Third Folio included seven additional plays, but only one – “Pericles, Prince of Tyre” – is believed to be by Shakespeare.