You can download the same book to multiple devices, usually at least 5 devices but that depends on the publisher. However, each device must be registered and a copy of the book downloaded to that device. You cannot transfer a DRMed book directly from one device to another because each copy has a DRM specific to the device on which it was downloaded.

So go to your Manage My Kindle web page, register your device if you haven't already, select the book you want to download, select which device you want to download it to, and your done. Or in the case of an iPod touch, if it is connected to a WiFi network and is registered with Amazon as a Kindle book reading device, you can download the book directly from the touch. Go into the Archive and select the book to download.