Tuning tablet into e-book reader?
Aside from the usual battery-saving tips (like disabling data / Wi-Fi), you could use an app like Screen Filter to reduce the brightness of the device; this reduces power consumption. 

If available, you might also invert the colors in your reader app. For example, ezPDF Reader has a Night mode feature which makes the background black and the text white (as opposed to the standard black on white). Of course, this is only applicable to PDFs with actual next, not scanned images.