I have two suggestions, which may improve the book.
1. It would be nice if the book would mention the fact that the CPUEmulator runs *a lot* faster if you choose "no animation". This information is especially useful for the Fill.asm project in chapter 4. I think as of now this information is only available on this forum.
2. In the section describing the syntax of .asm files, it would be nice to explicitly mention that inline comments, i.e. comments in the same line as instructions, are not supported. While the current description is correct and complete, I think assuming support for inline comments may be a common mistake. This is especially important because as of now, inline comments lead to a silent failure; the program simply won't load into ROM, without providing any pointers to the source of the problem.