KATONAH, N.Y. — On a cold, clear January day in 2008 Esmeralda Santiago walked downstairs to her office in her hilltop home here to immerse herself in writing the final pages of “Conquistadora,” her new novel. She was a week away from the deadline for this epic about her beloved Puerto Rico featuring a big cast of characters that includes the slave-driving, sugar plantation queen of the title, one Ana Cubillas. But when Ms. Santiago opened the computer document of her manuscript that day, the words looked something like this: Agttt Higg Bowq Sm. Pvxef byiz alwb.

“I’m thinking I must have been really, really tired when I wrote this,” Ms. Santiago recalled. Her next suspicion, based on her symptoms, was that she had suffered a stroke. The self-diagnosis was confirmed by a neurologist the next day. It ended up taking her about 18 months to relearn to read and write again in English. She is still struggling in Spanish, her first language.

This nightmarish detour was in some ways familiar, as it reminded Ms. Santiago of what it was like to learn to read and write English when she came to New York from Puerto Rico at 13, in 1961. “I went to the library and went to the children’s book section, and I started exactly the same process I did when I was learning English, connecting that word to that object,” said Ms. Santiago, who writes in English. Her novel was published this month by Alfred A. Knopf.

At 63, and with her long salt-and-pepper hair tied up, Ms. Santiago has a nearly unlined face and laughs easily and often. An open, fluid conversationalist, she was recently interviewed in her home, where she lives with her husband, the filmmaker Frank Cantor, and where they raised their two children, who are now grown.