As an add-on/interesting note, if you didn't mind a little more bulk, or if useage was going to be less, it would actually make sense to use a low self-discharge NiMh battery. The "best of the best" discharge at about 1.5% per month and even most consumer grade chemistries (like Eneloop) discharge at more like 3-4% per month.
So if you wanted a battery with a rechargable chemistry, a lithium Ion battery is likely to use between 10-20% of its battery charge through self discharge in 2 months of ereader use if expected use cycle was going to do that. A really good low discharge NiMh battery with a similar capacity (which yes, would be larger/bulkier for the same capacity) would only use about 3-6% in those same 2 months, actually giving you about 4-8 days more use out of the device.
If you had a device that had an expected duty cycle of say 4 months, suddenly you are now talking 20-40% lost through self discharge of the lithium ion battery and only 6-12% from that low self discharge NiMh, and now you are in to 2-4 weeks of extra use (14-28% longer).
Supposing the extra bulk of the NiMh battery was worth while it really starts to pay off in longer battery life due to lower self discharge (and rechargability was desirable, that dumb lithium ion battery, for example like they type you see in key fobs/remotes, LED book reading lights, etc don't have a voltage monitoring cicuit as they are not considered rechargable batteries, so they have effectively no self discharge).