I wanted to note that "What are you saying about the book" is also (not surprisingly) an accepted solution. If the meaning of the sentence concerns ideas about its contents, or even more physical things like its dimensions, then this translation using the present progressive sounds more natural to me. (We could also say, "What did you think of the book" to elicit reactions from the person we're speaking to (who presumably read it), and the meaning would be about the same.) On the other hand, "What do you say about the book?" could also have a quite different meaning, such as, "Should we donate it to the library?" or "Do you want to buy it?" With such meanings as these, it's this "What do you say" version, and not the "What are you saying" version, that's more likely to be used. So say my U.S.-American ears, at least.
Well, if lingots are involved -- : )
Kitap - hakkında - ne - diyorsun?
(The) book - about - what - are you saying?
For a discussion of hakkında and other "postpositions":
I thought this said something like "The book is about what you're saying?"
I find it strange how it is "what you say" even though it has "iyor" in it suggesting it would be "saying" i.e. continious present tense and not simple present tense. But I guess you are asking about what they would currently say about the book and not what they typically say about the book?
"The book is about what you're saying": Kitap senin dediğin (şey) hakkında. you need to learn -idik suffix for that, it is down the tree.
And the tense thing is because of different usages in both languages. In Turkish "kitap hakkında ne dersin?" is weird in my opinion, "ne dersin" is a phrase used for example for asking for a suggestion. "Sinemaya gidelim mi, ne dersin?" (Shall we go to the cinema, what do you say?)
In English of course one can use "what are you saying" and we accept it, but it is more appropriate to use "what do you say" to ask for the opinion like here