The books come from the LibriVox library, a non-profit project that has volunteers read the books and then releases the free recordings.
The app is ad-supported, but the ads are not obtrusive in my view. The books I listened to were competently read. Downloading was quick over WiFi,
Downsides are that every chapter starts with disclaimers about the book being in the public domain, which kind of breaks the mood.
The search function works well, but would be better if you could search by genre. There is a nice surprise me button, which gives you a book at random, but in practice that isn't really that helpful.
The developer of this app also sells audiobooks at the iTunes Store, which we have reviewed. These audiobooks show the text synced to the audio, but I do not find that a compelling feature. I think the new app is the way to go.
Here are a few screen grabs: