The book's main character is depressed, suicidal ideation and the main characters mental health status is a major plot point and he is actively considering suicide at multiple points throughout the narrative, coming very close a couple of times.
Those passages are very well written, Francis knew/understood - this is where this book got the 4.5 rating from, not the stolen horses plot.
But because of the inclusion of this type of content, the book may not be comfortable to read for e.g. someone who has lost a close loved one to suicide.