Just to correct your question first - you're asking about the difference between the present continuous and the present perfect continuous.

If someone asks you 'What have you been doing?' you answer 'I've been reading a book'. This is about a period of time which began in the past and has continued up until the present. It might be the last few minutes, or the last few hours, or the last few days. If you say 'I've been reading a book' , this can mean that you're still reading it now, or that you've just finished it.

If someone asks you 'What are you doing?' you answer 'I'm reading a book'. This means that you haven't finished the book. The question is about the present time: you were reading the book when the telephone rang.

"What are you doing?"...

"I have been reading a book" (OR "I've been reading a book") would be more factually correct compared to "I am reading a book", unless you are reading and replying on the telephone at the same time. ("I was reading a book" would also be acceptable in conversation.)

However, if you answered "I am reading a book", it could be acceptable as the other party would just think that when they called you, you were reading the book and the phone call interrupted your reading - acceptable even though some could argue that the statement is factually incorrect.

Is my explanation clear and understandable?

(P.S. Makes sure you don't talk to a "grammar nazi" over the phone; otherwise they will argue with you about the correctness of your reply, ha ha!)

Su.Ki. thank you very much

Berneddi M.Ed. Engl, thanks a lot.

Yes it is, the explanation is clear for me.