The author concludes that writers who write for the PURPOSE (”in order to”) of giving pleasure CAN impart truth to their readers.

The author tries to support this conclusion by showing that if this were not true, then that would lead to an absurd belief. To the author, if writers who write for the PURPOSE of giving pleasure couldn’t impart truth, then we’d be able to look at the sales figures of popular books, which we can assume gave people pleasure, and determine that some of what the books contain is false.

The author doesn’t complete the argument explicitly, but in the author’s mind, it’s obviously wrong to think we can determine that what a book says is false simply based on sales figures. This is why the author believes it can’t be true that writers who write for the purpose of giving pleasure cannot impart truth to their readers.

The author assumes that we can’t determine the truth of a book simply from its sales figures. (This is an assumption needed to complete the reasoning by absurdity. The author’s trying to show that if a certain belief were true, that would lead to a certain outcome that’s false. Here, the author needs to assume that that outcome actually is false.)

The author also assumes that if a book gave people pleasure, then the author of the book had the PURPOSE of giving people pleasure. (You can find this assumption from noticing that the author’s premises refer only to books that “gave people pleasure,” whereas the conclusion concerns “authors who write in order to give pleasure.”)