I'm repeating myself, but the letters are one of the jewels of literature.
As far as the review goes, I'm always disappointed at how many of these pieces struggle to get past the cliche of Flaubert as shut-in, ignoring his North African sexfest and only skating over his endlessly dazzling intellectual chatter.
There is the book that the linked review is covering, The Letters of Gustave Flaubert by NYRB publishing, which covers a lot of his life.
Another is Flaubert-Sand: The Correspondence from 1993, which is a selection of letters between Flaubert and George Sand.
If you can read french, the Rouen collection is probably the most complete, but there are other french collections. If you're just looking to start with Flaubert in general you can start with Mme. Bovary, rightly regarded as one of the best novels ever. Flaubert's control over his sentences, the breathtaking style, is incredible to read.
As far as the review goes, I'm always disappointed at how many of these pieces struggle to get past the cliche of Flaubert as shut-in, ignoring his North African sexfest and only skating over his endlessly dazzling intellectual chatter.
For anyone able to read French, Rouen University has a magnificent repository of the letters, many with scans. https://flaubert.univ-rouen.fr/correspondance/correspondance...