@book { EinIntermediavergleichvonDirectMailingundEMail,
    title = "Ein Intermediavergleich von Direct Mailing und E-Mail",
    author = "Markus {Hölser}",
    year = "2009",
    publisher = "Diplom.de",
    address = "Hamburg, Deutschland",
    doi = "10.3239/9783836639088",
    url = "https://m.diplom.de/document/227409"
}