Yes, you should buy book
You can learn online or you can buy the book online.
I would love to share few way to learn PMP
Google - You can learn from any search engine. Put your query on Google and get the solutions
Video - You can learn from video, watch the video and get the hint
Article: Read informative articles
Forum or Community: - You can share your query there and get the result by expert
Online Tutorials: - You can practice on online tutorials and do the practice at home
If you have budget, I want to suggest that you should do the course from any well-respected institute like - Koenig Solutions