LONDON (AP) -- William Shakespeare died 400 years ago, but his stock has never been higher.

To coincide with the anniversary of the Bard’s death, Christie’s is selling copies of the first four editions of his plays -- a collection the auctioneer’s head of books, Margaret Ford, calls “the holy grail of publishing.”

The four folios are going on display in New York April 1-8 and London April 20-28 before being sold in London on May 25. 

The First Folio, published in 1623, collected 36 plays, 18 of which had not previously appeared in print and would otherwise have been lost. It also cemented Shakespeare’s status as a serious writer, worthy -- unusually for a playwright of the time -- of a proper, leather-bound book.

Later editions, published in 1632, 1664 and 1685, helped preserve his work; the third also included a further play: “Pericles, Prince of Tyre.”

Of 750 First Folios that were printed, 233 are recorded as having survived. The one being offered by Christie’s, valued at between 800,000 pounds and 1.2 million pounds ($1.1 million and $1.7 million), is not on the official list. Christie’s says it and two of the other folios are from a “discreet and off-the-radar” private collection in Europe and haven’t been seen in public for 200 years.

The later folios are less valuable, but Ford says “you can count on one hand” the number of collectors who own all four. The volumes are being sold as separate lots, though Ford thinks a wealthy collector might be tempted to try for the whole set.

“It could be a coup to be the first person to bring Shakespeare (folios) to China, for example,” Ford said.

These days, the Bard is a global brand. The anniversary of his death on April 23, 1616 is being marked by events around the world, from a festival in Chicago to performances in the playwright’s home town of Stratford-upon-Avon. Shakespeare’s Globe theater is nearing the end of a two-year tour that has taken “Hamlet” to almost 200 countries around the world.

“There are so many truths there, and he does seem to speak to the ages,” Ford said. “I find it hard to imagine a world in which Shakespeare is not front and center in world literature.”