In the past few months, library-friends-group book sales, nonprofit internet libraries — even — have been flush with books that were, until recently, sitting on local Boston Public Library shelves. But why?

It’s housecleaning time at the Boston Public Library, with tens of thousands of books being pulled from branch shelves all around the city. And it’s a beast of a task. The only library in America with more volumes in its collection than the BPL’s 19 million-plus is the Library of Congress. Take that New York!

"The number of volumes in a library is not a good reflection of the quality of the collection or how well it's being used," said Larry Neal, head of the Public Library division of the American Library Association, and probable Yankees fan.

"The intention is not just to hoard everything that you can possibly stash away," he said. "Your job is to provide things that the community wants that have an interest in and make it easy for them to find."

And that means, sometimes, books have to go — something that Amy Ryan, head of the Boston Public Library, says is standard, daily practice for all libraries.

"That is our responsibility," she said. "And then, how do we keep our collection refreshed, up to date, accurate and responsive to the public?

The BPL adds more than 130,000 new books to its collection each year. And since books take up actual space, there is that pesky law of physics to consider.

Library studies have demonstrated that a shelf that is overly packed is unappealing to a user. That’s true of a retail store, too. But how do you decide which ones leave the shelves?

"The obvious one is damaged books," Ryan said. "Out-of-date medical books, out-of-date test books, out-of-date computer manuals — those leave the shelves."

But many of the books being weeded are perfectly accurate and in good shape. They just aren’t being checked out. The BPL is targeting books that haven’t circulated for four to six years. And this has some concerned, like Dave Vieira, a longtime patron and former President of the City-Wide Friends of the Boston Public Library.

"None of the books in the research library circulate, that doesn’t mean those books aren’t being used," Vieira said.

And consider patrons like Roxbury’s Brian Nally, who says he visits the central branch two to three times a week, but never checks out a book.

"I looked up the topic of meditation," Nally said. "Found a book that looked like it would help me, then sat in the courtyard and read it for a while."

Ryan says that officials at the BPL understand this, and are leaning on branch librarians to keep relevant, non-circulating books from getting the ax.

"Even if a book hasn’t circulated, but the local librarian says ‘I know that the fourth graders are going to come in and use these books on ancient Egypt, but they’re not going to check them out,’ that’s where the story behind the numbers comes in."

And the numbers do feel large. BPL officials point out that some of the some upwards of 200,000 titles being culled this year are simply something called database corrections that are books not even on the shelves. And they are also playing a little catch-up, having admittedly not weeded as aggressively as they could have in recent years.

But there is another thing happening here. Some branches are being asked to reduce at higher numbers than others as the BPL works to bring their neighborhood locations into the 21st century.

"So it’s really a complex equation that goes into making up a good branch library," Ryan said.

Today, libraries aren’t just about books. Where shelves once stood, there are now reading areas, computer terminals, and space for English-as-a-second-language classes. Ryan points to the new East Boston branch — which has the system’s smallest collection of physical books and the highest circulation — as the new model for a successful neighborhood BPL branch.

"Now the equation is changing as we turn to the public and work with public about what are their demands," she said. "What they do want is access to technology — so it is about computers, it is about books, it is about programs. So in terms of the choices, the allocation of the space evolves as peoples needs change."

But what Ryan calls an evolution, feels like a revolution to Vieira — one that he worries could leave loyal, older patrons behind. At the opening of the East Boston library, he says he couldn’t help but think, “Where are all the books?”

"While the library of the future may look like that, we are moving too fast, we are rushing from point A to point C," he said.

But whatever the speed, libraries, like the rest of us, have no choice but to trudge into the future. And for Neal, that future, looks remarkably like the past.

"Libraries were known as the peoples university in the early 20th century, and it’s interesting to see that role come back, in the way that people are learning in so many different ways nowadays, that the library really can be there to facilitate that life-long learning."