With “Harry Potter and the Deathly Hallows,” the seventh and final installment in the bestselling series having just hit the streets in record-setting numbers (each preceding book reached No. 1 on the New York Times bestseller list), J.K. Rowling’s ability to astound in the marketplace has become a given. Barnes & Noble reported pre-orders for the new book at 1.2 million in the U.S., the largest in the chain’s history. In its first 24 hours on sale, 8.3 million copies were purchased, topping the previous record (also held by Rowling).The author already had sold more than 325 million books worldwide and is, according to Forbes magazine, one of just five self-made female billionaires — and the first billionaire author. With all the hype centered on Rowling’s commercial appeal, it would be easy to dismiss her as simply a popular novelist. And yet she was named the greatest living British writer in a poll conducted by the Book Magazine — ahead of such esteemed writers as Salman Rushdie, Harold Pinter, Ian McEwan and Kazuo Ishiguro. Her celebrity is a far cry from her days as a single mother living on welfare in a cold-water flat in Edinburgh who often wondered if she’d have enough money to feed herself and her infant daughter. But the fame that has accompanied Rowling’s runaway success has not always been easy for the former teacher. “I cannot really emphasize how unconnected I was when all this happened to me,” she said last year. “It was very alien to me and I was scared rigid.” While it might appear that Rowling’s good fortune came with a few flicks of a magic wand, she’s been writing since age 6 (“I haven’t stopped scribbling since”) and her Harry Potter epics were meticulously planned long before the author had any inkling they’d become pop-culture gold. “She knew what the very last word of the last sentence of the last book was going to be from the beginning,” says Lisa Holton, president of Scholastic’s trade and book fairs. “What she’s done to generate an enormous excitement and enthusiasm for books and the power of reading is truly extraordinary.” Vocation: “All I ever wanted to do was be a writer.” Recent breakthrough: Completing the final Harry Potter book. Role model: Jessica Mitford, and her late mother Anne. (“Not a day goes by when I don’t think of my mother.”) What’s next: A thank-you tour in October, where she’ll visit schools in New York, Los Angeles and New Orleans; and a new children’s book, a “political fairy story” for readers younger than her usual Harry Potter fans.