Nuptial Law is a book which Klaus uses to figure out Count Olaf's plan. In the books, Klaus hides it under his shirt after the hook-handed man came to Justice Strauss's library. In the TV series, Klaus takes it without hiding it after Count Olaf offers Strauss a position in his play. It is the same book that Klaus uses to correct the marriage play and helps Violet to not "sign with her own hand",  which does not allow Olaf to obtain the Baudelaire Fortune. 

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