This is one of Gaarder's most remarkable books. It isn't long - it only takes a couple of hours to read - but the subject matter is probably the most mature and thought-provoking of his books, with the possible exception of "Through a Glass Darkly".
The backstory is that the author has discovered the manuscript of a letter, written from Floria Aemelia to Augustine, the Christian philosopher, and author of Confessions. Floria, it turns out, was the lover who bore Augustine a son, and who was abandoned by him as he immersed himself in his religious beliefs. The letter weaves together the story of their relationship, from their first to their last meeting, and also provides a response to Augustine's approach to human love. Floria has learnt much in the years they have been apart, and she frames her letter as an echo of the Confessions, in addition to filling it with allusions to other classical literature and the Scriptures that Augustine himself loves.
Floria argues with Augustine that for him to deny their love was to reject something good that God has made. She also highlights the way in which Augustine had been unable to break away from his relationship with his mother - and that in actual fact his response to Floria has as much to do with this distorting effect as his religious convictions.
It would be easy to think - especially reading this with modern eyes - that Gaarder, in presenting this parable, is endorsing the approach to love which says little more than "if love feels good, it is right". However, I think the argument is more subtle than that. Floria and Augustine, although unmarried, were effectively living in a relationship committed to one another, and already had a child. A more appropriate response for somebody troubled with this from a religious perspective would be for him to have married Floria. Certainly breaking one committed relationship to (as Augustine's mother hoped) form one considered more appropriate - forming a second physical bond in its place - is not a step forwards. And certainly for Augustine to have allowed his emotional life to have been ruled by his mother is also inappropriate. Further, Floria, in dismantling Augustine's approach to love, is quite happy to show that what he was doing did not represent a sensible interpretation of the Scriptures which he used as his basis for doing it.
In actual fact, Augustine's approach - his denial of physical pleasure, of human relationships - has more in common with a dualistic approach, which separates the physical from the spiritual realms. This, of course, is not a Christian idea. For the Christian, God rules over both the physical and the spiritual. That isn't to argue that physical expressions of love are always right - that is not what Christianity says. But it is to argue that to deny any role for the physical expression of love - as Floria was arguing that Augustine did - is definitely wrong.
Incidentally, the story fits almost perfectly the basic Tragedy plot given by Christopher Booker in "Seven Basic Plots", with Augustine the person who fails to arrive at completeness and understanding ....