Heretical. Wilhelm presents incomplete offical Catholic teaching in many areas, often followed by discussion of why he believes a Catholic does not have to believe or follow those teachings. He never defends the Catholic teachings with which he obviously disagrees. He endorses dissent in issues such as the immorality of all abortions, ordination of women, the existance of Hell, the immorality of contraception, calling God "Father" and many others. Not a book anyone interested in authentic Catholicism has any business reading. The Catechism of the Catholic Church is where you find offical Church teaching.
Years ago, when I owned an earlier edition of this book, a Protestant friend opened it randomly to discover what was, I think, a heresy. I suggest avoiding this book.