If I'm reading a book now, I can speak :
"I'm reading an interesting book".
If I began to read that book a week ago and I have been reading it for several days and I haven't finished it yet, I can also say
"I'm reading an interesting book".
For an accurate definition I can add the word "now" at the end of the first sentence, but if I add the subordinate clause "since I bought it a week ago" at the end of the second sentence, I have to use the present perfect :
"I have been reading an interesting book since I bought it a week ago".
How can I write the second sentence using the present continuous, so that it can be understood that I'm not reading that book at this moment?