{I've been reading} vs {I'm reading} (present progressive vs p. perfect progressive)

I think both

I've been reading that book (lately)

and

I'm reading that book (this week)

are correct. But I struggle to find a difference in meaning. Is there one? Should I avoid one tense or the other?

EDIT: I know how the present-progressive is usually used, but it can also be used for things that are not currently happening. That's the point of the question.

1. present perfect continuous
for an action that began in the past and continues up to the present.

I've been reading that book = I started reading it and I don't still finish.

2. present continuous
• for an action happening now. (Around the present.)

I'm reading that book = This is what I'm doing right now.

• for something temporarily happening not necessarily at the moment of speaking.

I'm reading that book in the evenings.

• for arrangements in the future.

I'm reading that book next monday.

• Actually the present-continuous can be used for an activity you are not doing right now
– drM.
Commented Mar 8, 2016 at 14:58
• Well ok, but I already knew what tenses they are. I'm asking if there's a difference between the two, and if there is, what's the one preferred
– drM.
Commented Mar 8, 2016 at 15:05
• @drM. Both work for temporary situations. The difference is that present continuous doesn't take the action in the past, it takes it on the present. The present perfect continuous allows to continue the action that started in the past, or also, to describe the cause of something in the present: My eyes hurt because I've been reading that book recently. Commented Mar 8, 2016 at 15:08

Let's break apart the contractions:

I've been reading that book (lately)

means

I have been reading that book (lately).

have been reading is present perfect progressive. It emphasizes that you have been performing a continual action at a present time (lately).

I'm reading that book (this week)

means

I am reading that book (this week).

am reading is present progressive. This tense refers to a continual action that is currently occurring ("this week").

Edit:

Of the two tenses, present progressive tends to be more immediate than present perfect progressive.