Imagine the late fine! Back at last, the holy book that someone 'borrowed' from St Paul's Cathedral library in the 1800s For at least 150 years the book known as The Missal of Sarum has been missing from the library of St Paul's Cathedral, a hidden treasure trove behind the south-west tower of Sir Christopher Wren's famous building. The original library was destroyed in the Great Fire of London, but when it was rebuilt in 1712 no fewer than 2,000 volumes were bequeathed by the Bishop of London, Henry Compton, to start the new collection. The missal, which dates back to 1502, was one. But at some time during the 19th century, it disappeared. Officially listed as lost by a "Miss Shepherd," and recorded in a hand that dates the disappearance to the early 19th century, mystery surrounds the book's disappearance - as no corroborating record of such a person working at the cathedral has ever been found. While those working at St Paul's would never say it publicly, there is a belief it may have left the cathedral premises under the cassock of a light-fingered priest. Imagine their surprise, then, when the ancient tome was suddenly advertised for sale by the Law Society, as part of a wider summer sell-off of its Mendham collection - a treasure trove of rare Reformation-era bibles and religious tracts amassed by the family of a 19th-century clergyman. It is unclear how the work came to be owned by the family. Staff at St Paul's library had no idea the book was still in existence until it came up for auction at Sotheby's in June, and immediately set about recovering it. They examined the legal options, but were advised that the break in the book's provenance would prove insurmountable. After the missal failed to make its reserve price at the auction - the estimate was set between £15,000 and £20,000 - the library negotiated a private sale with the Law Society, with Sotheby's acting as an intermediary. As a result, the missing volume has finally been restored to its rightful place on the library's shelves. Although the cathedral would not reveal exactly how much it paid for the book, it said the bill ran to "thousands of pounds." A private donor is understood to have put up the money. The missal is particularly valuable as it is from the early years of printing and it also spans a crucial time in religious history in Britain. Simon Carter, head of the cathedral's collections department, said: "The Reformation happened after it was printed. Bits have been cut out and erased which reflect those changes. St Paul's being the first protestant cathedral, the library collection reflects a lot of reform. The library now comprises more than 21,500 volumes. Among the treasures are 27 works that survived the Great Fire - including a psalter that dates back to 1270 - but the jewel in the crown is the Tyndale Bible, one of only three in existence and valued at more than £2.5m. The return of the missal marks the start of a significant few years for the ancient library. It is to be restored and volumes digitised in a £2m project that, after the recent 15-year cleaning and renovation work, marks one of the biggest internal redevelopments at the cathedral since Victorian times. Those running it hope to open the less well-known areas of the building to many more visitors. Oliver Caroe, the surveyor to the fabric of St Paul's - the 18th person to hold the role that goes all the way back to Sir Christopher Wren - said: "We want to make it an engine of learning that is more broadly spread to a different demographic and age range; more focused on what St Paul's is and how it lives and breathes rather than just a place of scholarship. That's the big transformation we're trying to bring. He added: "There is so much in this room that can draw out a story of the cathedral as well as the contemporary story of how we came about it." David Ison, the Dean of the St Paul's, ordered a consultation shortly after his arrival in May last year on how the cathedral could catch the public's imagination in different ways. Opening up the gallery on the triforium level - the location of the library - to visitors was one idea. It is hoped that the restoration will also help raise more money through ticket sales for a building that receives no state funding and needs £5m a year to cover its operations. The library needs basic conservation, including deep cleaning, restoring and protecting the books, many of which are damaged. Work will be done on the stone, wood and structure of the galleries, which are sagging. The space will be improved for the librarian and scholars to work and allow more visitors. A permanent display space will be introduced in the gallery after the success of the Hidden Treasures tours this year. Mr Carter said: "We're hoping to put the library catalogue online next year, which will be the first time the whole catalogue will be available to the public. It will be a big development for us. Staff have already digitised all of Wren's drawings and put them online. Mr Caroe said: "Just dipping into the story of the missal, there is a narrative to share with the public about why the building is here, how the history is relevant now. It's just one of many objects that can tell a really exciting story.