Bookeen emerged as a respectable manufacturer in the early days of eBook reader. We're talking ancient history here... TEN years ago! *gasp*
Kindles were yet to become a thing, and the e-paper market was largely dominated by Sony's early offerings - with excellent hardware but proprietary and intentionally crippled software, in the great Sony tradition of ineffectually blocking anything that could plausibly be pirated.
When others started entering the market Sony employed their well-honed process of trying to ram crippled products down people's throats anyway and only facing reality and releasing products that actually worked much too late - because that had worked so well a few years earlier in selling more mp3 players than Apple and Creative.
In any case the market was left wide open to the competitors, who all had worse hardware but readers that didn't require you to jump through absurd copy-protection loops to read your ebooks - which was important because at the time most people were actually reading pirated content, digital delivery being in its infancy.
It didn't take long for Amazon to start flexing its muscles and developing the concept of the store-dependant device, but in the meantime a smattering of smaller manufacturers brought their own e-readers to market, and Bookeen was probably the most successful of them until Kobo came around. Then Amazon dropped the price of the basic Kindles to impulse-buy levels and that was that, but before that happened Bookeen actually made some decent readers.
I myself have a Bookeen Cybook Opus as my own e-reader; I bought it six years ago for 110 euro (on a special sale - it used to cost 150), use it daily and have never felt the need to replace it - it's very basic, but it's small and easily pocketable and does its one job perfectly fine.
Bookeen themselves still exist, but they can't be in good shape; their ebook sales service is basically unknown, their last reader was announced in 2014 (and even that didn't receive stellar reviews), they've vanished from offline store shelves and Amazon and Kobo have stolen all the market. I've seen some local rebrands being sold recently that look suspiciously like the Opus; I suspect Bookeen gets all its current profit from licensing its designs to rebranders, though even that can't be much because the rebrands themselves aren't actually cheaper than Kindles.