The 2nd entry, "GNU Collaborative ..." is a quite decent English-English dictionary.
You can also find links to other dictionary sources and other information here:
https://github.com/koreader/koreader...ionary-support
If you want to launch the dictionary on long-press on a single word, make sure this option is enabled in the top menu, Gear icon > Taps and gestures > Long press on text >.