he has just finished writing the book - quando se completa uma ação vc deve usar o present perfect
You can use present perfect for an action that "just finished" but the action should impact the present. If the action has no relevance for the present, use simple past.
There seems to be many words used for finish/end. Can we cycle through them in most sentences or are there rules for what type of 'cease' word to use at certain times?