Answers: (C), (F)

The key here is to recognize that the second blank is referring to the reception of the book--the reviews the book received from critics. (A) disastrous and (E) unnoticeable do not work in this context because if critics thought the book was disastrous, then it would actually make sense that few people bought the book. In this case, the reception of the book would not necessarily be unnoticeable. In fact, the critics' reception of the book may be very noticeable because the poor reviews correlated with, and may explain, the book's poor sales. The same logic applies to (B). Therefore, (C) captivating works well. (F) ironic complements this nicely.

Of course irony is a complex term, and there has to be a sense of something running counter to expectations. In this case, if critics rave about a book it should not be so unpopular with the public. There is a reasonable expectation the book does well. Since it did not, with "so few people purchasing," the critic's reception proved ironic.
(D) poignant, meanwhile, refers to the melancholy emotions that something evokes. The critics' reactions to the work probably did not cause feelings of nostalgia, sadness, or regret; this is unrelated to the sentence.