@luis_in_brief For general reading I like KOreader. It's very adaptable, and you can set refresh frequency. I've got Vivaldi installed for the little internet browsing I do on the Leaf, but there is a browser - Einkbro - that is optimised for e-ink.
@luis_in_brief Yes, do a long press on the book you want to open, which will give you a menu with a range of options. Choose "open with", then select the "Set as default" button in the bottom left hand corner, then choose the app you want.
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