LONDON -- Copies of the first four editions of William Shakespeare's plays -- dubbed the "Holy Grail of publishing" by Christie's auction house -- have sold for almost 2.5 million pounds ($3.68 million).

Wednesday's sale included a previously unrecorded copy of the First Folio that fetched 1.87 million pounds ($2.75 million), well above its pre-sale estimate of 800,000 pounds to 1.2 million pounds.

The First Folio, published in 1623, collected 36 plays, 18 of which had not previously appeared in print.

The folio and later editions published in 1632, 1664 and 1685 were sold as individual lots but all bought by an anonymous private American collector.

Christie's books expert Margaret Ford said it was "exhilarating" to sell the four folios in the year marking the 400th anniversary of the playwright's death.