I have couple of old Palm Os based PDA devices: Handspring Visor and Hand Era 330; Both of these are suitable eBook readers.
+Quite long lasting battery
+Can be used in daylight with no backlight
+Better than color display if you read lots of text-only books
-If you are in to comics; then buy iPad or Android tablet
-Nothing can beat good e-ink based eBook reader (really it's like paper)
-Usually have to convert your own books
Palm OS eBook Software: