The obvious answer to
Calibre Companion including a builtin reader, is to use the
calibre ebook viewer.
As part of the calibre v3 rewrite, calibre will finally gain an in-browser viewer that should tick most of the boxes for commonly-requested ebook features.
It also handles offline viewing via LocalStorage, and as soon as bookmarks/annotations/etc. are implemented, those will auto-sync via the calibre server.
...
Of course, the rewritten server is not quite there yet.