Translation:We are going to read the book.
That would be θα. As far as I understand it, θα/will doesn't really convey when something is going to happen, just that it is going to happen at some point in the future. πρόκειται να has a richer meaning, in that it signifies that something is going to happen very shortly, very soon. It doesn't translate perfectly to English, "going to" is OK, but the meaning is probably closer to "just about to", "shortly going to" etc. So I don't think the two terms are fully interchangeable. I'm not a native speaker, but that's my understanding of its usage.
Πρόκειται does not necessarily mean that something is going to happen soon or any moment now. It means that something is planned to happen, so it's (definitely) going to happen as far as we can tell. In that sense, 'going to' is really close, in comparison with a simple 'will happen'.
Thanks for the clarification D_. In my village, I guess the two concepts are kind of interchangeable, as things that "will" happen... often don't, or don't happen for a long time, at least. Things that are πρόκειται να, usually happen quite quickly.
θα can be a bit like the Arabic "inshallah" but πρόκειται να does indicate, yes, it definitely will happen, most likely quite soon, and it's much stronger than will/θα.