Still thinking about it, but I found it generally uneven. There are some superficial philosophical treatments and at least one very serious definition error early on in this one--although he can probably be forgiven for this because he's a popular writer on spirituality with a reputation for spiritual direction of a certain type, rather than a theologian or philosopher. The mis-definition, and the sloppy equivocation that follows, appears only in one short section and does not carry throughout the book. Again, it's more of a philosophical problem than a theological one.
The word "certainty" carries a specific technical meaning in Neo-Scholastic (manualist) theology and it's fairly obvious that this was the meaning being used here, although this is by all other indications, a book meant for the general population.
Toward the beginning of the book, there were some very good, very perceptive (I thought) parts about the realization of the existence of God by the honest seeker, and about practical matters involving the spiritual life. That is Fr. Dubay's usual subject matter, after all, and he's very good at that. He knows that. On the other hand, some of the other overly-mechanical and more universally-presented parts were less convincing, and became repetitive as the book went on. A lot of this book consisted of the usual justifications for the kind of moral theory that's been around for the last couple of hundred years (manualism). Before the modern era, this kind of thing might have been unrecognizable to Catholics at every level of scholarly achievement. It's not the gospel, needless to say. And not at all original or particularly insightful.
The last chapter looked like an afterthought, probably intended as an illustration of the previous chapter which was more metaphysical. That last chapter probably was somewhat unnecessary for postmodern readers, but perhaps not so much for the readers of the late 90s, when this book was new.
Overall, there are much better things to read on this topic. I was required to read this for a class and so I persisted until the end.