Not the parent and I've never read that particular book, but the premise seems rather bizarre to me, where is the excitement in implementing a slow version of Java in Java? If anyone is going to want to implement a custom language, it will likely be very different to Java, e.g. Lisp, the core of a DSL or configuration language. But it's still going to probably have functions and data types. Writing Lisp in C is exciting, one is moving up the abstraction hierarchy, bilding a more expressive and powerful language.

If one wants to learn specifically how Java works, then maybe a book on compilers would be better?

You might not be excited about “implementing a slow version of Java in Java”, but that doesn’t make it a bad approach. (Also, second part is about implementing same interpreter in C)

Well implementing all the OOP semantics and such could complicate and dilute the essentials. A simpler language would also make it easier to cover more ground, such as type checking. So I don't think it sounds ideal from a pedagogical perspective either, but again, I haven't read the book.

Java in Java isn't slow, it is a matter of how it is done.

JikesRVM, MaximeVM and now GraalVM.

I agree but the context was interpreters. GraalVM is a compiler, it JIT compiles bytecode into machine code.