Looking for research on enewsletters

I had a call this morning from a reporter looking for research on the growth of enewsletters. He's working on a piece for a US based in-flight magazine and is at the preliminary stage of looking for stats and research about enewsletters.

Any suggestions/recommendations?

(Be interesting to see what impact (if any) blogs have had on enewsletters).

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[K McGoey]

Marketing Sherpa - Email MArketing Benchmark Guide 2006
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