What is the difference between these two sentences, one in the present continuous and the other in the present perfect continuous?

If we use your question as an example:

1 I am reading a book this week - I started reading it a few days ago. I haven't finished it yet.

The present tense implies a current action that is in progress as you speak and will continue into the future. It minimises the emphasis on the reading up until the present.

2 I have been reading a book this week - I started reading it a few days ago. I haven't finished it yet.

The past perfect refers to the time that was spent reading between opening the book for the first time and the moment before the sentence was spoken. It allows for the possibility that you have stopped reading it without actually stating that possibility.