New York’s public libraries want about $1.5 billion from the city over the next 10 years so they can fix their buildings, which are old, crowded, falling apart or lacking in things they need to be useful in the 21st century, like electrical outlets. They also want more operating money, $378 million, up from about $320 million, to improve programs and services in the coming fiscal year and to stay open longer — a basic indicator of library-system vigor in which New York lags embarrassingly behind Chicago, Los Angeles, Dallas, Phoenix, Philadelphia, San Antonio and San Diego. And the city’s suburbs. And Albany.

Mayor Bill de Blasio should give them the money, no question.

Library funding has been lagging for years. The city’s three systems — the New York Public Library, which covers Manhattan, the Bronx and Staten Island; the Queens Library and the Brooklyn Public Library — were left struggling to do more with less under Mayor Michael Bloomberg, and need a huge infusion of money to stabilize themselves, make overdue repairs and return service to where it was before cuts dating to 2008. The city’s libraries are privately run institutions that depend heavily on public funds — overwhelmingly so in Brooklyn and Queens, which lack the deep philanthropic pockets that make mid-Manhattan’s landmark library gleam. Keeping the sprawling system healthy is a core municipal responsibility.

Mr. de Blasio should act because his city is demanding it, more than ever. That is, the people whose needs are the core of Mr. de Blasio’s mayoral mission, whose priorities he trumpets, immigrants and the poor.

The libraries are where poor children learn to read and love literature, where immigrants learn English, where job-seekers hone résumés and cover letters, and where those who lack ready access to the Internet can cross the digital divide. Libraries can be a natural fit for mayoral projects like after-school programs and prekindergarten, and for the city’s justly lauded municipal ID program. They are havens for thinking, dreaming, studying, striving and — for many children and the elderly — simply for staying safe, and out of the heat.