With most phones barely having enough juice to last a day, backup batteries have become a demand lately. The difference may lie on the size, bulkiness and of course power stored, all depending on of course on how often you would be using your device.
For the size part, maybe this one would interest you. The Vorson Bookmark is said to be something slim while packing 2500 mAh of backup battery power. The device is only 4.7 millimeters thick.
It passes of juice through the expected USB interface, something you will find at the bottom part. It also comes with a touch-activated battery meter so you can check how much energy is left on it.
Costing about $60, the Vorson Bookmark should be a reliable backup power storing device, packing energy said to be enough to charge larger devices such as the iPad Air.