Many wondrous surprises lurk inside public libraries. Literary marvels. Decades of archived newspapers. Endless volumes of storybook fantasies. But at one Bay Area library, it’s a delicious Vietnamese sandwich that’s stoking imaginations and appetites.

Prologue Coffee & Tea has been slinging fresh, thoughtfully prepared banh mi inside the Walnut Creek Library since the cafe opened in June. Proprietor Thuy Nguyen left a 20-year career in banking to open the cafe, which is accessible via the library, as well as a separate entrance on North Broadway. Prologue is named for the beginning of this new chapter in Nguyen’s life, but it’s also a love letter to her daughter and a way for Nguyen to share her flair for Asian-inspired drinks like dalgona matcha, banana milk coffee and a charcoal rose latte with Japanese activated charcoal.

“I was just always intrigued by making coffee and tea drinks for my family and friends,” Nguyen recently told SFGATE. “It was a hobby. I never really thought about opening a coffee shop.”

That hobby flourished into a business when Nguyen’s daughter reached school age and she felt like she didn’t have time for her while working an 8-to-6 corporate job. “My purpose is my daughter,” she said. “It’s in the library, and I can bring my daughter here, and she can read........

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