@book { GrundlagenPortrtfotografie,
    title = "Grundlagen Porträtfotografie",
    author = "Alexander {Spiering}",
    year = "2017",
    publisher = "Schlütersche Verlagsgesellschaft mbH & Co. KG",
    address = ", Deutschland",
    url = "https://schluetersche.openpublishing.com/document/404139"
}