Amazon has a marketplace here:
http://www.amazon.com/mobile-apps/b?...ode=2350149011 The download link is on the right.
The Google Marketplace is here:
https://market.android.com/?hl=en It is what most people mean when they refer to the marketplace.
Both have lots of apps, many free. Amazon has a free app every day that normally costs.
As for which app is best for ebooks, this is a matter of taste. A lot of people like Moonreader, and lots of others like Coolreader. There are several others, and both of these sites should have them.