I also like eReader. The basic version is free, and you can mark bookmarks and search for specific words. There is a Pro version that costs, it has additional features. Check the details at
http://www.ereader.com/product/browse/software. You can also get a desktop version so you can read books on your handheld and on your computer.
I've found ereader to be easy to use, and there are more books available every month from a wide variety of categories. You can also buy the software to create your own ebooks that can be read on ereader.
I have used Microsoft reader, but it was such a pain to register I don't want to rely on it in the future :bang: .