Should I write 'I have been reading a book now' or 'I am reading a book now'? I started to read one week ago and I haven't ended yet. Thanks for help
It depends on what exactly you want to say, of course.
In most cases the present perfect continuous has an associated length of time of the action.
I have been reading a book now for a week.
The present continuous only conveys the fact that you're currently engaged in some activity. If you don't care to relate how long ago it started, use present continuous. For instance, you want to say that the book is about some topic you just touched in the conversation:
I am reading a book now on this exact subject.