@book { AcomparisonofthemainDirectMarketingMediaandtheirfutureprospectsintheageofthenewmillennium,
    title = "A comparison of the main Direct Marketing Media and their future prospects in the age of the new millennium",
    author = "Roman {Keilhacker}",
    year = "2000",
    publisher = "Diplom.de",
    address = "Hamburg, Deutschland",
    doi = "10.3239/9783832424138",
    url = "https://m.diplom.de/document/218225"
}