Matejka, Michal, Sandra Onderstal, and Anja de Waegenaere. The Effectiveness of Caps on Political Lobbying. N.p.: CentER, 2002. Print.
Feuer, Alan. For Ex-Lobbyist Abramoff, a Multimedia Effort at Redemtion. N.p.: n.p., n.d. Print.
Bealle, Jackson. Why lobbyists negatively influence the US legislative process. N.p.: n.p., 2007. Web. 11 Nov. 2011. <www.helium.com>
Most of my sources i found using the advanced search options in Google. First I searched "political lobbying", then "positive" or "negative." Using advanced search option, I decreased the amount of hits by more than half.
