The root of these six words is:

k – t – b

ك – ت – ب

Now look at the English equivalents for these words. Remember that with nouns the context finally decides what is actually meant. 

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

Remember that the root s – l – m conveys the idea of ‘peace’.

What would you say the root k – t – b conveys?