\documentclass[a4paper,oneside]{book}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage[american]{babel}
\usepackage{csquotes}
\usepackage[style=authortitle,backend=biber]{biblatex} % ,sorting=ydnt
\usepackage{hyperref}
\addbibresource{biblatex-examples.bib}
\DeclareBibliographyCategory{primary}
\DeclareBibliographyCategory{secondary}
\addtocategory{primary}{aristotle:anima,aristotle:physics,averroes/bland}
\addtocategory{secondary}{hyman,moraux,nussbaum,pines}
\defbibheading{primary}{\section*{Primary Sources}}
\defbibheading{secondary}{\section*{Secondary Sources}}
\begin{document}
\chapter{References by category}
This is just filler text.\footcite{aristotle:anima}
This is just filler text.\footcite{nussbaum}
This is just filler text.\footcite{averroes/bland}
This is just filler text.\footcite{hyman}
This is just filler text.\footcite{aristotle:physics}
This is just filler text.\footcite{moraux}
This is just filler text.\footcite{pines}
\printbibheading
\bibbycategory
%
% \bibbycategory processes the categories in the order in which they
% were declared. Note that the category name also serves as a heading
% identifier, i.e. you need to define a matching heading for every
% category. In this document, \bibbycategory is a shorthand for:
%
% \printbibliography[heading=primary,category=primary]
% \printbibliography[heading=secondary,category=secondary]
% ...
\end{document}
Oups, je me suis complétement planté ! (la chaleur sans doute) Si j'ajoute
,sort=ydnt
(ce que j'ai fait chez moi) cela ne fonctionne pas, mais avec la syntaxe correcte,sorting=ydnt
(celle de la question), cela fonctionne parfaitement. Désolé pour le bruit.