I have used RS dictionary and also a Merriam-Webster's and WordNet dictionary, which I could make available to you. They are basically the same, though. A much better option is to install KOReader for Pocketbook and download some of the more comprehensive dictionaries available through the software itself.
It's best to use this stable test build of KOReader, it is being adjusted for Pocketbooks at the moment:
https://github.com/koreader/koreader/pull/6541