I've found that the Kindle app for Android is the best out there, hands down. I have two different Android tablets. One is a Galaxy Tab 7". The other is Android running on my laptop. Since I run Ubuntu and Android on the laptop on the same partition, it's quite easy for me to import books on the Kindle app for the laptop. Android-x86 (the name of the project that runs Android on PC) uses the hard disk it's installed on as the sdcard (there is no need for the external sd, since I already have 160 GB on the internal), so all I have to do is mount the partition as root, copy the ebook files to data/media/kindle and when I boot into Android and launch Kindle, I see the book on the homescreen. It's actually very simple.
On the Tab, I connect it to the PC and launch Calibre, which detects the Tab as an Android device and I can use the Send To Device button to send the books I want to read. It imports into sdcard/kindle folder. I'm not sure how it will show up on Windows, because Linux uses mount points to mount USB devices. That is, they appear under /home/media/Galaxy \Tab. Windows uses drive letters, like I:/, which points to the location of the device.
When I tried importing the books into the external sdcard, the tablet couldn't find the files. This is a problem with Kindle for Android app and would encourage you to report it as a bug to Amazon.
Note, this only happens when the internal is mounted as the sd and the sd card is mounted as an external sd. Meaning, it only happens to devices that have Android version 3.0+