Thief does research, finds hidden passage to secret library and steals books. Old but good.

I’m afraid my burning passion overrode my conscience. It may appear selfish, but I felt the books had been abandoned. They were covered with dust and pigeon droppings and I felt no one consulted them any more. There was also the thrill of adventure – I was very scared of being found out.

Book thief explains why he stole 1,100 ancient books in a hidden room in a monastery library. The theft and the sentencing happened several years ago but just popped on to my radar and resulted in my finding another neat source of library/security information, the library theft category fo the Museum Security Network blog. Current posts are password protected but they become available after a while. Here is their post on what drives people to steal precious books.
Details of the secret room didn’t make the major media, but can be found in this Masters thesis on bibliokleptomania

“While some structural details are uncertain, many accounts agree on certain aspects of Gosse’s methods. The journey inside required him to scale a sheer exterior wall, which led to an attic workshop, which is now part of the hotel. From there, he followed a disused corridor to the next building. At the end of the corridor, he climbed down a very old rope ladder to a small sealed room. By pushing on a portion of the wall, he discovered that a bookcase inside the next room gave way. He then found himself standing inside the library.”