There are a dozen different e-book reader apps out there for android. There are several that people swear by, and many more that really aren't even worth trying out.
Here is a list of my top e-book reading apps in order of awesomeness (functionality and usability).
Aldiko is a great app, its powerful, and feature rich. What makes Aldiko really great is that it allows you to add ebooks that you've purchased from different sources, or made yourself, and not forcing you to buy all of your books through their market. Aldiko also supports checking out books from your public library.
The reading experience is customizable allowing for several font and background color combinations for both day and night modes. Resizing of text and margins is also supported.
For the time being Aldiko doesn't include any cloud features such as syncing books and reading position across multiple devices, or a web-based reader.
Booki.sh is the new guy on the block and its design and implementation reflect current trends and advancements in web app design. Booki.sh is a web app designed with HTML5. Cool, what that mean. Well for users that basically means that with Booki.sh the app works whether you are online or not. Simply go to Booki.sh with your favorite browser and your books will download and become available to you when you are offline. Offline reading is exactly the same as online reading, just return to the Booki.sh website and everything will work exactly the same as if you were online.
Booki.sh also has several customization option for the reading experience, including day and night modes and font choice.
Booki.sh easily add and syncs your books, bookmark, and reading position across all your devices; Android, iAnything, Firefox, Chrome, and Safari. DRM-free and .EPUB books bought elsewhere are easy to imported into Booki.sh, you aren't locked into their market, as you are with some of the other big apps.
Booki.sh runs in your browser so it takes a little longer to open than other apps since the browser has to open first, and then load the Booki.sh web app.
Kindle is a solid e-book reading app from Amazon. The big draw to this app is the built-in Amazon market allowing easy access to tons of books. Kindle is a fine app and a little more basic than Aldiko, however it does allow for syncing of reading position across your devices. You can read books not purchased through the Amazon market with Kindle, but the app won't sync them to your other devices.
You might not want internet retail giant Amazon looking over your shoulder as you read, and it doesn't have as many options as Aldiko.
Google Books feature list are almost the same as Kindle's: cloud syncing of books and reading position between devices. Some reading experience customizations such as day and night modes, and text tweaking. Google Books also has a huge built-in marketplace for purchasing new ebooks.
Google Books has no support for reading books not purchased from Google. So your either stuck using two reading apps, or you'll be buying your books again just to use them in Google's app.
In my eyes it's either Aldiko or Booki.sh. If you have only one device and books collected from all over the place its Aldiko. If you have multiple devices that you read on or your own collection of ebooks, its Booki.sh. However if for some reason you find that you need