Both the Confessions and the Essays are didactic autobiographies; but while the author of Confessions sees his life as a drama illustrating the linear movement of consciousness from egocentricity to theocentricity, the author of the Essays sees his life as a dispassionate, varied exploration of the innumerable styles of being a self.
I largely agree with this view of the Confessions, except that they culminate in these "timeless" reflections on memory, time, and creation. These latter reflections almost force you to reread Confessions with them in mind, and see them throughout. As such, while Confessions has the appearance of such linearity, it really becomes a feedback loop. I have to admit I need to read more of the Essays to get a much better sense of them: they are about 1000 pages or so. But the styles of being a self, as Sontag puts it, are not discretely separate styles. They are organically linked in multiple ongoing microscopic processes of physical and mental transformations of the self, the self in the processes of imaginative possibilities and inevitable decay.