I really, really like HaaliReader -
http://haali.cs.msu.ru/pocketpc/ I converted the free 1914 webster dictionary into haalireader dictionary format and now have a huge dictionary lookup facility :-) I used to use starbuck (which I bought) but haalireader is so much better. I use it every day and have done for a couple of years. The documentation is not great though.
I like the idea/feature of ubook but I always use haalireader when it supports the format of book I have (I can get more text on screen with a small font and cleartype disabled).
The other option is mobipocket.com which has both a free basic reader and a pay "expert" reader.
All comes down to taste/the format of the books you read. Haaliereader is very configurable.
Chris