I wouldn't read too much into it. His comments about cryptocurrencies in the past show that he has no stake in crypto and didn't spend much time getting acquainted with the technology. My guess is that he either received the book as a present or is simply catching up with recent developments.
He may be an investor of Cryptocurrency, you never know! Elon Musk is a smart man and a visionary leader. While the whole world is slowly acknowledging the power of Cryptocurrency, he may want to know about it with a future business model. Cryptocurrency is slowly getting into our daily lives and powerful leaders around the world are acknowledging it. It's hard to ignore!