I read on average 2-3 hours a day and I only use the sleep mode and have been getting around 14 days going from full charge to no bars.
You are getting 12 days going down to 2 bars and reading 3-5 hours when you use sleep mode? either I have a rubbish battery or you have a great one.
Im not sure what difference how many books on your reader makes but I have around 550 on there.
If I delete most of them will I be more likely to double my battery time???