It only works with MobiPocket books.
I know it's expensive, but how often do you buy a dictionary? This is the first one I've bought in close to 30 years. If this one lasts equally long, I'll consider it a bargain
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The nice thing is that you can use it on the Windows desktop Mobi reader too, and there it has very sophisticated search facilities - wildcard searches, etc. On the Gen3/iLiad it can only be used for looking up words in a book, not searching.