I assume you are aware of the Nokia 770 and N800. They are exactly what you want except with a color LCD screen and (therefore) a battery life measured in hours. I have never formally measured the battery life, but it is at least 4 hours reading e-books (wifi turned off). It has a true standby instant-on mode, and if you use it every day it is entirely possible that you would never turn the device off. Extra Nokia batteries are available if you want to run all day, but there is no external recharger so juggling multiple batteries is a pain.
The N810 is rumored to be available shortly, and this has already caused price cuts for the N800. The 770 is actually a better e-book reading device that the N800 (better key layout), but is a bit underpowered for web browsing etcetera.