Do you read with a pen or pencil and write in the margins as you go? Readers have been doing this as long as there have been manuscripts. Annotating the books we read can greatly enhance our enjoyment and memory of them!
Beyond simply underlining, here are some suggested symbols you could use to strengthen your own engagement with books 2 and 3 of Confessions:
* this is interesting to me
† deep, spiritual, inspiring
? I have a question about this
// This parallels something I have read elsewhere
∞ This connects to something else Augustine has said
! what a surprise
!!! say what!?
x I disagree
T this appears to be a theme in Augustine
wp this looks like an example of word-play
x3 Augustine expresses something in a triad
Agree, marking up the book is necessary since there is SO MUCH amazing content!