Google's lesser-known Java libraries, such as Guice, Error Prone, Truth, Kythe, and Protobuf, offer significant benefits to developers by enhancing productivity, code quality, and interoperability. Guice stands out as a lightweight dependency injection framework that simplifies code maintenance, while Error Prone helps catch programming mistakes at compile-time to prevent bugs. Truth provides a framework for more readable and informative test error messages, and Kythe offers tools for interoperability among systems manipulating source code. Protobuf, known as Protocol Buffers, is a language-neutral, platform-neutral system for serializing structured data, praised for being smaller, faster, and simpler than XML. These libraries, along with others like AutoValue and GSON, offer various functionalities that can make Java programming more efficient and effective, beyond the well-known Guava library.