Actually, I think there's something else going on here - batteries nowadays drain themselves over time - i.e. leave a rechargable AA battery out for a few weeks, and it'd be down to 50% or less.. The battery on the sony reader might just be draining as well.
My personal experience is that I lose my first battery bar after one book - but then it hovers around the 2-3 bar mark until I cave in and recharge it.
-mark
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