Twenty-five years after it released one of the most successful books in its history, Chronicle Books will publish the final title in Nick Bantock’s Griffin & Sabine series next month. Griffin & Sabine: An Extraordinary Correspondence, published in 1991, is an epistolary novel featuring the correspondence between two unlikely lovers: Griffin Moss, a London artist who makes postcards, and Sabine Strohem, an artist in the South Pacific who illustrates stamps. The saga, told in a unique format of illustrated postcards and removable letters between the fictional characters, developed a cult following, and now more than three million copies of books in the series—which includes Griffin & Sabine, Sabine’s Notebook, and The Golden Mean—have been sold. Chronicle will release the last book in the series, The Pharos Gate: Griffin and Sabine’s Lost Correspondence on March 22, along with a special 25th-anniversary edition of Griffin & Sabine: An Extraordinary Correspondence, which includes a newly created envelope with a special postcard and commemorative stamps.

Bantock initially created the first title as a project for himself. “I lived on a very small island off the coast of Vancouver, and every day I would go and get the mail, but it was always junk mail,” he said. One day Bantock noticed a man next to him at the post office “getting this great-looking letter in a sweet blue envelope. I thought that’s just not fair. I left the post office feeling sorry for myself and realized that my best chance of getting any decent mail was if I wrote to myself.” As he walked home, he had notions of two people writing to each other, and by the time Bantock got to his studio, he had the whole scenario for Griffin and Sabine sketched out in his head. Because Bantock thought of himself as an artist and not a writer at that point, the book prominently featured illustrations and artwork. But he also added the distinctive envelopes affixed inside with the characters’ letters. That distinction really set the book apart. “There’s something about opening an envelope,” Bantock said.

He started to build an initial draft of the book, but had no intentions of publishing the title until it was discovered by accident when he was visiting Chronicle to pitch children’s book ideas to Victoria Rock, then the company’s children’s publisher. He just happened to have a dummy copy of Griffin & Sabine in the bottom of his bag, where Rock spotted it. “She asked if she could show it to somebody, and when she came back she asked to hold it for a week.” Chronicle called saying they wanted to do the book.

Then-publisher Jack Jensen, who is now Chronicle’s president, took the book to ABA (now BEA), with plans for a modest print run of 10,000. There was some hesitation among the Chronicle staff about publishing a title with such a unique format, and how to do so at an affordable price. But the book was met with great enthusiasm by booksellers at ABA and the print run was upped to 30,000.

Chronicle publisher Christine Carswell and Bantock both attributed the success of the first titles in large part to the role played by independent booksellers. “The books are a great hand-sell,” Carswell said. “Not only that, but they are a great hand-read as well, because you have to open the envelopes and remove the letters. Everyone had their own intimate response to it, and I think part of that has to do with physically opening those envelopes.”

Bantock’s original wish to receive handwritten mail like his neighbor came true in spades. “After the first month, I started to get huge amounts of mail,” Bantock said. “It was really interesting, because the age group was incredibly broad. It seemed to touch different people on different levels for completely and utterly different reasons.” One fan letter came from Kurt Vonnegut, who praised the series in a handwritten letter to Bantock calling Griffin & Sabine “a masterpiece in four dimensions.”

The new book, which reveals the fate of the couple, has an announced first printing of 50,000 copies. Bantock said he first thought about doing a final book in October 2014. After his agent approached Chronicle about the new book, the publisher asked him if it could get it done by October 2015 to release it to coincide with the 25th anniversary of the first book. “I had under six months to finish everything,” Bantock said. “But that was no hardship, because it was in every sense a labor of love.”