I keep Encarta pocket dictionary installed all the time. If you are reading an E-book with MS Reader, you can "tap-hold" on any word, and look it up in this dictionary from the drop-down option. Used this way, the dictionary finds the word amazingly quickly. Its tight integration with MS Reader makes it easy, and I find it look up words I should but wouldn't bother with if it weren't this easy.
However, as a stand-alone product, the Encarta pocket dictionary sucks. That's because there is no good way to find a word except throught MS Reader.
There are plenty of dictionaries out there. My favorite is
Lectionary