Underlining a word was a very serious offence

As for the library, the regulations that had to be followed were singularly severe. Books could not be taken out of the factory under any circumstances; they could be consulted only with the consent of the librarian, Signorina Paglietta. Underlining a word, or just making a mark with pen or pencil, was a very serious offence: Paglietta was expected to check every book, page by page, when returned, and if she found a mark, the book had to be destroyed and replaced at the expense of the culprit.

Primo Levi, The Periodic Table, (Phosphorus),  Penguin 2000, p.94 (first published 1975)