So began the inspired fictional correspondence between Griffin Moss, a London postcard artist, and Sabine Strohem, a South Seas postage-stamp creator. Published last fall with little fanfare, the book became a favorite among the nation’s booksellers, who voted it one of five books they’d most enjoyed selling in the past year. Word of mouth kept it alive (it stayed on the San Francisco Chronicle’s best-seller list for 47 weeks), and it has now sold a whopping 175,000 copies. Gearing up for the just-released sequel, Sabine’s Notebook (Chronicle. $17. 95), Chronicle ordered a first printing of 225,000 copies.

Part romance, part psychological mystery, “Griffin & Sabine” and “Sabine’s Notebook” are among the strangest books ever published (a third, concluding volume is due in a year). Strictly speaking, they are old-fashioned epistolary novels, written in the form of letters between two talented, lonely people who fall in love via airmail. But there’s no strictly anything about these books. Sabine, it transpires, may be a mere figment of Griffin’s overheated imagination. More intriguing, these are picture books for grownups, and like the lyrics and melody of a song, the art and the prose are completely intertwined. Taken singly, the words and pictures are interesting but not arresting. It’s when they’re put together that things become both beguiling and unsettling. Ranging from dadaist collage to Leonardolike pen-and-ink drawing, the pumped-up visual imagery tartly counterpoints the romance’s wispiness, and advances the plot. The art styles of the two correspondents, radically different in the first book, begin merging into one style as the lovers draw closer in “Sabine’s Notebook.”

“I don’t remember ever picking up a book where the words and pictures had equal validity,” says the 43-year-old author. “Either you were dealing with an annotated story line or you had an illustrated book. What this meant for me was that there was never a chance to hover between the real world and the unreal. One of the things I was determined to do with these books was create this strange oscillation that you get in the evening or early morning, just before you drop off or just before you wake up, where many things are far more acceptable.”

The most striking thing about these books is that they aren’t just leaves between covers: the letters are actual letters tucked inside envelopes glued to the pages. Like the rest of Bantock’s work-he’s an inspired creator of pop-up books for children-the letters in “Griffin & Sabine” and “Sabine’s Notebook” are visually delightful and wonderfully tactile as well. Reminiscent of antique oddities like stereopticons, they hark back to a time before telephones. They demand to be handled, rifled, used. His creations ably scratch Bantock’s itch to “push at the edges and boundaries of what is perceived as a book,” but they do it by looking backward to the hands-on era of letter writing and the centuries-old paraphernalia of pop-ups.

Mail-poor no more, Bantock is now flooded with fan letters. His favorites are “the letters that say, ‘The book made me get my paints out again after 10 years’ or,‘I started writing a friend.’ That the book has encouraged people to make and do things themselves, that I find exciting.”