in addition to
teaching about
compilers, he has written several
books on the subject, which include:
Having taken a class that used the ML book, I can vouch for its quality: very good to learn from, if you read a chapter, and implement the program as it suggests. As a reference book, it isn't so good, although it does have some nice summaries of advanced compiler techniques and computer architectures. (apparently the C and Java versions of the book are almost identical to the ML version, with C/Java code snippets that try to act like ML)
(what that tells you about Appel himself, I don't know.. that he is knowlegable about compilers, probably a good teacher, or at least that he can write good assignments?)