The author of the best-selling book series said she had been undergoing treatment for glioblastoma, an aggressive brain tumor, after a diagnosis in 2022.Sophie Kinsella, the best-selling English author of the “Shopaholic” book series, revealed on social media on Wednesday that she had been undergoing treatment for an aggressive and often fatal form of brain cancer.Kinsella said that she had been diagnosed with glioblastoma in 2022, but waited to make the diagnosis public so her children could “ hear and process the news privately and adapt to our ‘new normal.’” She added that her condition was stable after a successful operation and ongoing chemotherapy and radiation at University College Hospital in London.Kinsella, whose real name is Madeleine Wickham, has written a string of hit novels, starting with “Confessions of a Shopaholic” in 2000, about a financial journalist in New York City with a serious shopping addiction. About a decade later, a movie starring Isla Fisher based on the original novel and a sequel was released.Since the smashing success of the first novel, nine sequels following the life of the protagonist Rebecca Bloomwood have been released, earning Kinsella, 54, a loyal following and a reigning position among authors of romantic comedy books.Kinsella said that readers’ response to her latest novel, “The Burnout,” had “buoyed” her during a difficult time undergoing treatment. The novel, about a couple of worn-out office workers who meet at a dilapidated British seaside resort, was published last year.Glioblastoma is an extremely aggressive brain tumor. There is no cure, and most patients do not survive beyond one and a half to two years. “It’s such a terrible, devastating disease,” said Dr. Wajd Al-Holou, a neurosurgeon at University of Michigan Health. The condition is relatively rare; the National Brain Tumor Society estimated that more than 14,490 Americans were expected to receive a glioblastoma diagnosis in 2023.Doctors typically try to remove as much of the tumor as possible in surgery, and patients also receive chemotherapy and radiation to try to slow the growth of the cancer. The tumor often grows back.Glioblastoma most frequently occurs among people between the ages of 50 and 70, Dr. Al-Holou said, and is more common in men than women, for reasons doctors do not fully understand. Doctors are also not certain what causes gliobastoma.“It’s completely random, it’s out of nowhere,” said Dr. Viviane Tabar, chair of the department of neurosurgery at Memorial Sloan Kettering Cancer Center.Symptoms can develop rapidly and vary depending on where in the brain the tumor is located. People can experience headaches that become more severe over a short period, blurred vision, weakness in an arm or leg, difficulty speaking, memory loss, nausea and vomiting and seizures. Sometimes, people’s personalities seem to change, Dr. Al-Holou said.Kinsella’s longtime publisher, The Dial Press, an imprint of Random House Books, did not immediately respond to a request for comment.