Here's my followup to using the Calibre reader:

You have to install the whole Calibre app on your PC. However, you can definitely open just the reader without opening the main app by creating a shortcut to ebook-viewer.exe.

The reader is okay, but just. It is slower to open than the other readers I've used, and it tends to become temporarily non-responsive a little too often. Probably bloated code.

I don't like the lack of a margins setting to adjust how the book appears on screen. This results in partial lines being displayed at the bottom of the screen. Setting a margin at the bottom would perhaps alleviate that.

You cannot set the text color or background color.

In tablet mode, switching the LCD from portrait to landscape or vice versa causes a temporary not responding message.

In landscape mode, there is no two-pages view. On a 9" tablet in landscape, you really need this two-page view.

Overall I found it usable but not enjoyable. It seems bloated and slow on a netbook, lacks some basic features, becmes unresponsive often (though it always seems to recover without a crash). It definitely looks and feels as an afterthought app rather than a dedicated reader app. I've now uninstalled it from my T91MT but will definitely keep it on my main desktop to use for converting and adjusting eBooks.