First-Person Narrator (Eva Khatchadourian)
The first line of the book shows us that Eva is writing a letter: "Dear Franklin, I'm unsure why one trifling incident this afternoon has moved me to write to you" (1.1).
As the epistolary style develops, we slowly learn that Franklin is Eva's husband, that they are separated, and that they are separated because—spoiler alert—Franklin is dead. That's the permanent separation.
The narrative style allows Eva to examine details of her past and see if they illuminate anything about her present. When trying to find what went wrong, Eva has to sift through her entire life looking for little nuggets of wisdom. The letters do that, and they allow us to follow her on her mental journey.
Also, Eva is a writer. She may have written travel guides, not self-help books and psychological analyses, but she is still a writer. And the reason she wrote travel guides was because she "wrote the guide that I wished I'd been able to use myself" (3.32).
So whether or not Eva learns anything from her letters, maybe someone else will by reading them. Could have Eva done anything to stop Kevin? Probably not. But just as she wrote travel guides to prevent others from making the same mistakes she made, this book of her letters serves as a travel guide to parenting.