The Zsun wifi smart card reader is a stamp-sized, cheap microSD card reader that sells for as little as $11 (or a couple bucks more if you want to order from Walmart or Amazon rather than runthe risk of buying from a Chinese retailer). It's designed to be plugged into your computer's USB port and act as a flash drive while at the same time serving up files to devices connected to its Wifi network.
It's a pretty nifty gadget, but when it is fresh out of the box you have to have Zsun's app to access the files over Wifi. That works, but now hackers have found a way to improve on the original design.
Designed to run routers and other embedded devices, OpenWRT is open-source and is not for the faint of heart. Installing it can bork a piece of hardware, but once you've successfully installed it you will have additional control over a router's settings and performance.
OpenWRT is used to power the LibraryBox file server mentioned in the second link in this post, and you can use it for a similar purpose with the Zsun card reader. And if you plug it into a USB powerpack, you'll get a truly portable and pocketable ebook server:
But again, this project is not for the faint of heart. From Liliputing:
On the other hand, installing OpenWRT on the Zsun card reader is kind of tricky, but you can find detailed instructions at the project’s Wiki page. The most foolproof methods involve some hardware hacking, but there are some software-only solutions… it’s just that they’re very risky and if something goes wrong you could kill the device.
I've killed a router while trying to install OpenWRT, so I would advise caution.