The letters are included as an addendum to the biography.
<there's an addendum from the author to explain certain stylistic choices that she made>
Francine du Plessix Gray, in an essay first written in 1991 and updated with an addendum here, writes eloquently of a lifelong obsession with weight loss, stemming from the harsh judgments of a mother enraptured by fashion. —Ginia Bellafante, New York Times Book Review, 21 Sept. 2008
This qualification, however, is not a recent addendum to Derrida's career. In 1990, near the end of the period in which rhetoric and composition had a strong interest in his work, Derrida participated in an editor's interview … in which he characterized his project as fundamentally tied to the tradition it works both within and against. —College English, September 2006