For almost four years, hundreds of thousands of books belonging to Manchester have been stuck down a salt mine in Cheshire, patiently waiting for the city's Central Library to reopen after a £50m makeover.

Many were priceless treasures: Shakespeare's Second Folio from 1632 and a handwritten copy of the Codex Justinianus, a 12th-century code of law originally compiled for the Roman Emperor Justinian.

While the manuscripts were in temperature-controlled chambers 1,600 feet (500 metres) below soil, planners, architects and builders were working above ground to bring the magnificent Grade II*-listed neoclassical structure back to its former glory.

On Saturday. the doors of the revamped complex will finally open.

Manchester Central Library celebrates its 80th birthday this year, but looks much older. The original architect, Vincent Harris, was reportedly inspired to create its striking rotunda form by the 2nd century Pantheon in Rome. Though loved by most, it has always looked as if it perhaps landed in modern Manchester by mistake, having got lost en route to Italy or Greece a few thousand years ago, hemmed in by the neo-Gothic splendour of the town hall on one side and the Edwardian baroque red bricks of the Midland Hotel on the other.

Harris made one important concession to the environment, however, building the library from durable Portland Stone, which copes well in even the rainiest climes.

By the time the library closed in the summer of 2010, the domed roof of its once glorious reading room was riddled with asbestos, Photograph: Christopher Thomond

The biggest public lending library in Britain after Birmingham's, it was opened in 1934 by King George V, who described it as a "splendid building" which offered "magnificent" opportunities for the city.

But by the time the library closed in the summer of 2010, the domed roof of its once glorious reading room was riddled with asbestos and the warren-like layout of gloomy corridors and forbidding stairwells had become a no-go area for the younger generation of readers.

"In the old library, 70% of the building was not accessible to the public. We've reversed that so now 70% is open," said Neil MacInnes, head of libraries at Manchester city council.

Targeting young readers is key: there's even a "gaming area" with Xboxes and Playstations, as well as a children's library modelled on The Secret Garden (written by local lass Frances Hodgson Burnett) in the 20,000 sq ft of new space carved out underneath the town hall extension.

From the outside, the library looks the same, but step in through the main "Shakespeare's entrance" – which features a painstakingly restored stained glass mural depicting some of the bard's best-known characters – and it feels like a new building. Instead of being greeted with a brick wall and stairs, visitors enter an open-plan atrium packed with interactive displays on local history and a cafe selling Lancashire crisps, Goosnargh cakes and Manchester fruit loaf.

A glass wall on the far side gives a glimpse into what the old library was like – narrow rows of metal shelving housing dusty old books. Look up through a new glass window and you see the 300-seater reading room on the first floor, one of the few computer-free parts of the new complex.

The restored reading room is undoubtedly the jewel in the crown. Acoustic engineers have worked hard to reduce the echo that used to amplify sounds so much so that "if you shut a metal door nearby it sounded like a car bomb going off in here", Roger Williams, Manchester city council's press officer, recalls. Retained is the quote from Proverbs 4 which has always encircled the dome: "Wisdom is the principal thing therefore get wisdom: And with all thy getting get understanding. Exalt her, and she shall promote thee. She shall bring thee to honour when thou dost embrace her. She shall give to thine head an ornament of grace; A crown of glory she shall deliver to deliver to thee." The new design celebrates the mundane as well as the sacred: one display case contains sweet wrappers from the last 80 years, all found stuffed down the desks by peckish visitors sneaking a sugar rush during their reading.