12-15 books would still be huge. I'll believe it when I see it. So far every reader I've had in my hands didn't last for a week without recharging. I'm lucky if I can get through one book, I'm pushing it for two, and three is entirely impossible. Of course since I read with a comfortable font size (6-8 words per line) I have at least twice as many page flips as someone who likes to have the same squinty-eye feeling that paper books offer... but still.
You have to consider that reading 15 books on this thing requires you to transfer books three times, and I assume bluetooth will suck as much energy as wifi. The 12-15 books figure is either based on small books, small fonts (fewest pageflips possible), or is just a theoretical value based on pure pageflip count, not taking standby times into account. Same way any other company is overly optimistic regarding their battery times.