But really, it should have a 'sleep' option. Cold booting each time...
Actually there is already a sleep mode: The device is in sleep when the actividy LED is off! The E-Ink display doesn't need any energy to keep showing a page, just for flipping pages. The buttons and the G sensor create an interrupt which wake up the device from sleep.
When you activate the keyboard lock as kacir suggested, the G sensor will be turned off too, so you have the deepest possible sleep mode (like the S3 "suspend to RAM" mode on a PC).