Here’s a quick redirect to a Library Juice post with a couple of nice resources.
First is this guide for collecting from diverse sources.
(or: outsourcing, how not to)
Fostering Media Diversity in Libraries: Strategies and Actions.
Second there’s a link to a note on the ALA’s opposition to media concentration in the US since June 2003.
Relevant Canadian stuff from libraryland (found by searching the CLA website) is largely falling under the information literacy umbrella:
School Libraries in Canada link.
Information Literacy in Canada blog post.
-PC-