Rescuing the BPL's rare books is a job of jaw-dropping proportion. Including those on display, the library holds half a million titles and a million manuscripts. The most valuable aren't the ones on view on glass-fronted shelves. Interested in The Bay Psalm Book, the first book printed in North America? The library's got two. Want to see the handiwork of Johannes Gutenberg, the father of modern printing? The BPL has a page from his Bible, and one of the few copies of his Catholicon. Shakespeare? Unless you have a really good reason, you probably won't get to see the famed 1623 First Folio, the original collection of his plays. That one's kept in a vault.

Most of the rare books arrived in the late 19th century, back when Boston was the Athens of America, back when Bostonians were building a new home for their public library (the nation's first), founded in 1848. Vivian Spiro, chairwoman of the Associates of the Boston Public Library, a group that raises funds for the library (and to which, full disclosure, I belong), puts that 19th-century giving into 21st-century perspective. "The optimism that made the creation of the BPL possible," she says, "is now responsible for its predicament regarding the conservation of its most important holdings."

When the library's Copley Square building opened in 1895, founding elders and wealthy Brahmins gave their money and their books. But their benevolence has created a huge challenge for their successors. Flush with good intentions, those early patrons never thought about having to survey, clean, and eventually repair their donated volumes. Decades later, the gargantuan task of upkeep is a responsibility the city is struggling with. "We're not doing a very good job," says Walker.

The outgoing library president agrees. "In some ways, we're cursed by having so much," Margolis told me recently. After Mayor Tom Menino orchestrated his ouster in November, Margolis accused him of being "anti-intellectual" and of putting the library's treasures in danger. That enmity hasn't dulled. "It's a lot to maintain, and sometimes we've needed leadership that understands that investing in this resource as a public good produces a direct economic return. But we haven't had the blessing of that."

Given the library's limited resources, deciding which books to target for restoration is a scattershot process. "We've been forced to move from a proactive model, where we identify and treat important collections, to a reactive model, where our work is essentially need-driven on an item-by-item level," Walker says. Sometimes he's alerted when a patron requests an old book. The volume emerges from a century of slumber in the stacks, flops open, and falls apart; Walker adds it to his pile. But he tries not to contemplate the thousands of books that cry out for his attention. It's too disheartening. He focuses instead on simple satisfactions of the work. "I like to take things apart and put them back together," he says. It's good that he's enthusiastic, considering how little help he has and how important his work is, not just to the city, but also to the cultural heritage of the nation.

To save a book, you must first take it apart. It's not a sight a book lover wants to see. "There is nothing more violent and upsetting," says Prindle, than watching Walker unbind a volume.

For a hardcover book, like the Priestley, whose covers will be replaced, Walker clutches the pages in one hand, as though grabbing a chicken by the neck, and simply pulls the cover off. He wants to expose the backing, the place where the glue holds it all together. The adhesive must go, not only because it's in the way, but also because it's eating the paper. Walker applies a poultice of methyl cellulose to the exposed backing. The compound breaks down the glue, leaving a gooey muck he then scrapes off with a dull knife.

Next, he separates the pages into their gatherings, the four-sheet bundles of paper that compose the meat of the book. He cuts the ancient linen threads that hold the gatherings together, then lays flat the individual sheets to be washed. For a book like the Priestley, whose pages are made of linen, the culprits Walker needs to wash away are the acidic animal glues as well as the gelatinous compounds that were spread over the paper to fill its pores. In younger books—meaning those less than 200 years old or so—the acid he battles is left over from the papermaking process itself. Around 1800, manufacturers started making paper from wood pulp, which involves using acids to break down the cellulose fibers in the source timber. The thing about acid is, it never quits. Walker says lots of books printed between 1800 and about 1970 are cooking away in the stacks. Some even smell burnt.

Before he bathes the Priestley's pages, Walker has to consider the chemical properties of the ink used for the marginalia. In Adams's day, people often made their own ink, relying on recipes that seem equal parts witchcraft and household chemistry. Two centuries later, the variations in the quality and acidity of these concoctions can surprise even an experienced conservator. Walker has never lost any marginalia, but he did once put a printed sheet of paper into a water bath and watched everything "just lift off the page." Sentences disintegrated, words came apart, letters went floating about like alphabet soup.

To test the ink Adams used to write his notes, Walker places a drop of deionized water onto one of the president's bits of marginalia. ("This sentence is a string of sophisms," Adams had noted to himself.) The ink doesn't break apart. Next, the scarier part: seeing whether it will withstand total immersion. He puts the page into a mesh-screen holder; once wet, it will become even more fragile. Then, with a steady hand, he sets the holder in the water tray. Walker likes to say that at this point he puts his faith in God and chemistry. He's well aware of the consequences, should either one fail him.

When the sheet hits the water, it dampens at the margins and along the strip that once met the spine—the spots on the page that are most covered in glue or most exposed to the air and the corrosive atmospheric particles that have been around since the dawn of the Industrial Revolution.

With the water gathering on the paper, Walker watches the sheet closely. "You always worry," he says. "Did Adams decide that this would be the page where he experimented with some new ink?" The sheet seems to hesitate. Then something happens. One of the handwritten notes begins to attract water. "Don't see that too often," says Walker, with the kind of detached calm that you hope never to hear from a doctor who's examining a funny growth on your leg.

But the ink holds fast. After a moment, Walker relaxes. "That's probably where Adams opened a fresh bottle of ink. Probably a slight change in acidity."

Slowly, the sheet surrenders to its fate and sinks. The air fills with a nutty odor as the glue (made from horses' hooves) and the gelatinous compounds break down, turning the water yellow. But the words of John Adams stay right where they are. God and chemistry have done it again.

Before he's finished, Walker will replace the water twice, rinsing the sheet three times. Later he will "paint" the pages with an alcohol solution called sizing that restores the stiffness, or as he calls it, the "crackle." After he has done all that a hundred times or so for the 450-page book, he will resew the gatherings, rebuilding the book with needle and thread. The whole project should take about 35 hours. But for Walker it'll end up requiring two weeks. Because of personnel cuts, his work is interrupted by other tasks—like supervising the rare books reading room while other staffers are at lunch.