Canis, great that you've dropped in on the thread! Congratulations on getting v1.0 out of the door, I'm just giving it a first trial run now.
First impressions: it has a nice, clean, OSX feel to it. Am I right in thinking it's basically a front end to Project Gutenberg? Which is fine. Here are some thoughts:
Navigation - it would be useful to be able to skim through a list of authors quickly, rather than skim through an entire list of works. Ditto genres, titles. The user needs to be able to browse easily, as well as search for a specific title or author (which you've implemented nicely).
Viewing - I really like the full screen mode. I'm not keen on the fact that it defaults to centering text which I think is counter-intuitive, surely it should default to left alignment or justify?
The speech option works well (another good reason for making it Leopard only, with the addition of an almost real-sounding voice). Not sure I'd ever use it though. Keyboard shortcuts to start and stop speech would be useful.
I'd like to see more viewing options - it would be nice if the user could configure font, font size, font colour, background colour, etc.
Most importantly, I think the real added value of your application will come when it can be used as a library or catalogue management tool in tandem with a dedicated e-book reader. It would be really nice to see conversion tools built into the application (text to .mobi for example), and syncing. I imagine that you're looking ahead to the iPhone but don't forget other hardware!
At the very least, something you could implement immediately would be 'export to text'.
In short though I'm really glad there are Mac developers out there who see a market for this kind of application, I'm very interested to see how it evolves.