What book did Twilly send to Nick?

The correct answer is that Twilly sent Nick "HayDuke Lives" by Edward Abbey. This book is significant within the context of the characters and themes involved. "HayDuke Lives" is a novel that reflects on themes of environmentalism and rebellion against societal norms, which resonates with the character of Twilly. His choice of this book suggests a connection to the values he embodies and attempts to instill in Nick.

The other options, while notable works, do not share the same thematic relevance to Twilly's character or the story at hand. "The Adventures of Tom Sawyer" deals with childhood and adventure, "The Call of the Wild" touches on survival and primitivism, and "The Secret Garden" explores themes of healing and transformation, none of which directly articulate the rebellious spirit and ecological consciousness that "HayDuke Lives" conveys. This connection highlights why Twilly’s gift to Nick is particularly well chosen, emphasizing a shared understanding of the world’s issues.