1) Sometimes the colors of status bar and action bar are not matching (blue and red, etc.). It seems CalibreBox looks at the colors of the cover.jpg and decides what color should be applied, which is cool, but I think two different constrast colors look weird.
2) The download button is easy to understand, but the "open book" button is not. And can you make it clearer to identify which books have been downloaded, and which not? An option for automatic update of newer book files?
3) There is a black bar under the book list - It could be an ad, I don't know, because Adaway is present in my phone. I tried "Purchase Pro Version", but it says "No Internet Connection".
. (In fact, I do have smooth connection now).
4) Can you support custom columns as well?
5) I have no problem with Dropbox - which is fast. But if you can support OneDrive, it is also great because it comes built-in with Windows 8.x