My review of "Understand" is not actually of the audio book but rather of the story as it appeared in the August, 1991 issue of Asimov's Science Fiction. This review is excerpted from my review of that issue:
Whatever else they may be, stories by Ted Chiang are smart. "Understand" is about being smart, being, in fact, one of the smartest people who ever lived.
Leon Greco, the narrator, suffered extensive brain damage from oxygen deprivation. Doctors treated him with an experimental medication, hormone K, which regenerated damaged neurons, but then went on to do more, to continue to increase Greco's intelligence.
Greco agrees to take additional hormone K. He gets even more brilliant. Then things begin to change between Greco and the doctors working with him:
I realize who Clausen is: he's a government psychologist, perhaps military, probably part of the CIA's Office of Research and Development. This test is meant to guage hormone K's potential for producing strategists.
Greco, now certain that the government wants to use him, decides that his best course is "to downplay my skills and get [questions] wrong." Then he withdraws from the study.
Greco is sure that the government wants to take him into custody. He goes into hiding; he also steals another ampule of hormone K.
At this point, I think that it would be reasonable to wonder if Greco's suspicions are deserved or if he is suffering some mental instability, possibly as a consequence of hormone K. I don't know if that is what Chiang intended, though. Perhaps the reader is simply supposed to trust Greco's feelings.
Greco says that he gets increasing control over his body, using biofeedback techniques. Also, he says:
I acquire years of education each week, assembling ever larger patterns. I view the tapestry of human knowledge from a broader perspective than anyone ever has before; I can fill gaps in the design where scholars never even noticed a lack, and enrich the texture in places that they felt were complete.
And more:
I'm designing a new language. I've reached the limits of conventional languages, and now they frustrate my arrests to progress further.
Greco injects the stolen ampule of hormone K. Undreamed-of changes continue. Then Greco becomes aware that there is another being like him, a man named Reynolds. "I am a lover of beauty, he of humanity," says Greco. "Each feels that the other has ignored great possibilities." Could they work together or must they compete?
A fine story. In addition to being chosen by Asimov's readers as their favorite novelette to appear in the magazine in 1991, it was also nominated for a "Best Novelette" Hugo Award. When I first read it some years ago, I loved it. I still like it very much, but now I believe there are flaws. The first is intrinsic; to be consistent, the last section of the story should be in Greco's "new language." Obviously, this is not possible. Also, I honestly don't know whether Greco is intended to be sane or not, or whether that is even relevant.
Many of the people posting comments about "Understand" here on Goodreads mention this story's resemblance to a much earlier tale of greatly increased intelligence, "Flowers for Algernon" by Daniel Keyes. I believe that "Algernon" is a somewhat better, more moving story than "Understand."
And a quote from the story "It Opens the Sky" by Theodore Sturgeon:
You don't, you just don't increase intelligence by a factor of five and fail to see that people must be kind to one another.
I would like to believe that statement is true.