Trojita currently stores the address book in the ~/.abook folder, but it would be more consistent if it used $XDG_DATA_HOME/trojita/abook instead (at least when trojita runs in a XDG-compliant [1] desktop). [1]: https://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html
The background is that the original abook¹ has no actual XDG support (you can pass a config and data file on start, but it only looks up ~/.abook by default) Otherwise one would of course have used QSettings defaults anyway. [1] http://abook.sourceforge.net/
Oh I didn't know abook was an external project. Reported upstream: https://sourceforge.net/p/abook/feature-requests/13/
Closing as per comment #1