It can be difficult to determine the "best" book for Java, as it depends on individual preferences and needs. However, "Effective Java" by Joshua Bloch is widely considered to be a classic and essential guide for Java developers of all levels. It covers best practices and design patterns for writing clear, efficient, and maintainable code. It is a great choice for developers looking to take their Java skills to the next level.
Another popular option that is widely considered a classic is "Head First Java" by Kathy Sierra and Bert Bates, which is considered a great choice for beginners as it uses a visually rich format and a conversational style to teach the fundamentals of Java programming.
Additionally, "Java: The Complete Reference" by Herbert Schildt is also a widely recommended guide, it is considered as a comprehensive reference guide for Java programming which covers all the fundamental concepts of Java and its advanced features.