NEW YORK >> Six letters by "To Kill a Mockingbird" author Harper Lee to one of her closest friends could fetch as much as $250,000 at auction.

Four of the letters date from before "Mockingbird" while Lee was caring for her ailing father, Amasa Coleman Lee, the model for her protagonist Atticus Finch.

The signed and typed letters were written to Lee’s friend, New York architect Harold Caufield, between 1956 and 1961, according to Christie’s, which is selling them on Friday.

In one, she writes about her "stunned" reaction to the huge success of the book, published in 1960 and made into a movie starring Gregory Peck two years later.

"We were surprised, stunned & dazed by the Princeton review," she wrote. "The procurator of Judea is breathing heavily down my neck — all that lovely, lovely money is going straight to the Bureau of Internal Revenue tomorrow."

In another letter she tells Caufield: "Daddy is sitting beside me at the kitchen table. … I found myself staring at his handsome old face, and a sudden wave of panic flashed through me, which I think was an echo of the fear and desolation that filled me when he was nearly dead. It has been years since I have lived with him on a day-to-day basis."

The sale comes as Lee’s second book, "Go Set a Watchman," is set to be released in July. It was written before the Pulitzer Prize-winning "Mockingbird" but takes place 20 years later. Lee’s agreement to release the book stunned the literary world. "Watchman" has been ranked No. 1 in new releases of classic literature and fiction on Amazon.com for weeks.

"She’s arguably one of the most important American novelists of the post-war period who has not published a great deal," said Tom Lecky, Christie’s head of books and manuscripts. "She’s remained a private figure so the appearance of an archive like this is a very important moment in the marketplace. Only three other letters of hers have come up at auction within the last 35 years."

Christie’s said the seller, who wished to remain anonymous, acquired the letters on the open market. The auctioneer intentionally has blurred the contents in its catalog and online to protect the author’s privacy.

The 89-year-old Lee, who is in declining health and lives in an assisted-living home in Monroeville, Alabama, could not be reached for comment about the sale. Her attorney, Tonja Brooks Carter, did not return an email and phone request for comment.

In a 1961 letter Lee wrote that Esquire magazine turned down her "pastiche" about "some white people who were segregationists & at the same time loathed & hated the K.K.K. This is an axiomatic impossibility, according to Esquire!"

The letters, totaling eight pages, are signed as Nelle, New Hampshire, or with comic pseudonyms including "The prisoner of Zenda," a reference to an 1894 novel about a king who is drugged and kidnapped on the eve of his coronation.

The letters present "an interesting bracket of before and after of a monumental moment in American letters where you have the time leading up to while she’s writing her book and then you get a glimpse into her reaction to the reaction," said Lecky. "You see how the book was received and how she received those reviews."