Kindle, Sony and eBook readers: How many a month?

I'm doing a bit of research on the question of ebooks and the appropriate formats to use in publishing some research and archival content I have. Do you have an Amazon Kindle, Sony Portable Reader or another favorite eBook format (Please add your comments on devices and formats in TalkBack)? If so, how many titles do you purchase each month (that is, how many individual ebooks and subscriptions to newspapers or magazines)?

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