Let's do some math!

Assume that 20 pbooks have the environmental impact of 1 ereader, and assume that that calculation was based upon an average $10 paperback. Finally assume that the typical ereader costs $200 (less for eInk, but less avid readers will jump at the tabletesque ereaders).

Now $200 over 20 pbooks means that our assumed typical pbook should have a $10 tax. Convert that to percent, which is totally fair because bigger and more expensive books will have a larger environmental impact, and we see that we should be taxing pbooks at 100%.

Ereader fanatics of the world rejoice: the pbook would effectively be dead under that taxation policy!

(Note: In a fit of sarcasm I embellished the calculation by using a cheap pbook and an expensive ereader.)