What is the author's tone in Adrift: Seventy-Six Days Lost at Sea by Steven Callahan?

Adrift is a story of minimum necessity. Callahan can take nothing for granted. Everything he has, even common insignificant-seeming objects, is precious. Food and water must be rationed. Work must be balanced against exhaustion. This lends the story an air of desperation, as Callahan strains to husband his meager resources to meet the minimum requirements for continued survival. Should some critical component fail or become lost, Callahan is doomed. The tone is one of anxiety and apprehension.