The direct influence between lobbying and policy is often difficult to prove - but a good deal of evidence is available in what the lobbying groups reveal about their activities and in how the legislation gets crafted. It is important to look at both sides of the process, starting with the lobbying group or organization. You can check their web sites for statements about the policies they are working on - The American Library Associations Legislation Page for example. Then you can check the hearings and legislation versions in Proquest Congressional for changes recommended by the lobbying groups, etc.
For information on where lobbying money goes - try these resources:
For information connecting lobbying to outcomes try these sources: