I am not sure where blx@hook@citekey@next comes from in the example below.
The output of the footcite is:
1 blx@hook@citekey@nextCarl Menger. Grundsätze der Volkswirtschaftslehre. Braumüller, 1982.
\documentclass{report}
\usepackage[backend=bibtex,citestyle=verbose-ibid,bibstyle=numeric,sorting=nyt,defernumbers=true]{biblatex}
\DeclareBibliographyCategory{websites}
\DeclareBibliographyCategory{spoken}
\DeclareBibliographyCategory{books}
\AtEveryCitekey{
\ifentrytype{book}{
\addtocategory{books}{\thefield{entrykey}}
}
}
\usepackage{filecontents}
\begin{filecontents}{\jobname.bib}
@book{3,
author = {Menger, Carl},
title = {Grunds{\"a}tze der Volkswirtschaftslehre},
publisher = {Braum{\"u}ller},
year = {1982}
}
}
\end{filecontents}
\addbibresource{\jobname.bib}
\begin{document}
\chapter{Chapter}
Some text
\footcite{3}
\end{document}
filecontents
. Inexperienced users tend to overwrite files by acciddent. – Johannes_B Feb 22 '15 at 13:34