The reasoning clearly proceeds by the contrapositive, where the author concludes that X cannot be true, because if it were true, then Y would also be true. The tone of the author implies that Y is weird and not true.

The conclusion is that the claim in the first sentence is not true, or in other words, authors who intend to impart pleasure can also impart the truth. If that was not the case, the author argues, then books that impart pleasure would be partly untrue. Since sales, popularity and pleasure tend to correlate, the more sales a book has, the more popular it is and hence, the less “truth” such a book would impart. So we could determine the truthfulness of a book just by looking at its sales figures, which the author clearly believes is weird and not true.

The author basically states the weirdness of a situation to argue that the intention of imparting pleasure is consistent with the ability to impart truth.

(A)  When people choose to read a book, they generally do not already know whether reading it will give them pleasure. 
The opposite of this statement is an assumption. The author has assumed that sales correlates to popularity and hence pleasure. So people buy only when they know the book will give them pleasure.

(B)  Even when an author writes with the goal of giving people pleasure, that goal will not necessarily be achieved. 
Out of context. There is no discussion about how successful authors are in achieving their goal.

(C)  In many cases, a book's readers are unconcerned about the truth of the book's contents. 
Out of context. We are not discussing if readers are bothered about the truth or not.

(D)  A book will not give its readers pleasure 
unless it was intended by its author to have 
that effect. This option is quite interesting. I initially opted for it just by eliminating the others without even thinking through it. The conclusion is about "authors who write in order to give pleasure". But the reasoning about sales of books and popularity is general and not restricted to just "authors who write in order to give pleasure". So the two of them will tie together only when it is assumed that only authors who intend to give pleasure will give pleasure. Other authors will not.

(E)  A book can be popular for reasons other than 
its ability to give readers pleasure. 

The opposite of this statement is an assumption. The author has assumed that sales correlates to popularity and hence pleasure. So popularity implies pleasure for the readers.

It has been said that authors who write in order to give pleasure cannot impart to their readers the truth of their subject matter. That claim cannot be true. If it were, one could determine the truthfulness of a book simply by looking at its sales figures. If the book were very popular, one could reasonably conclude that it gave people pleasure and therefore that at least some of what is written in the book is not true.

Which one of the following is an assumption required by the argument?

(A) When people choose to read a book, they generally do not already know whether reading it will give them pleasure. 
- Out of scope ( we are talking about not about what people choose to read)

(B) Even when an author writes with the goal of giving people pleasure, that goal will not necessarily be achieved. - Out of scope (authors goal has not been discussed) 


(C) In many cases, a book's readers are unconcerned about the truth of the book's contents. - Extreme Language 


(D) A book will not give its readers pleasure 
unless it was intended by its author to have 
that effect. - CORRECT 


(E) A book can be popular for reasons other than 
its ability to give readers pleasure. - Out of scope (reasons are not discussed)