The root k – t – b conveys the idea of writing.
Look again at the words. Can you see how they are related to the idea of ‘writing’?
كتاب kitaab – document, manuscript, book, etc.
كاتب kaatib – writer, author, secretary, etc.
مَكت maktab – office, desk, etc.
كتب kataba – he wrote
يكتب yaktub – he is writing / he writes
مكتبة makataba – library