I don't know what the situation is like for Kindle, but there are atleast two open source alternatives for Kobo. KOReader is the Swiss Army knife, Plato is easi

I don't know what the situation is like for Kindle, but there are atleast two open source alternatives for Kobo. KOReader is the Swiss Army knife, Plato is easier to use. A few people are suggesting that KOReader is symptomatic if the issues with open source software. The reality is it targets a particular group of users.