The calibre e-book viewer is useable as a standalone program on all operating systems, including macOS (there is a separate ebook viewer bundle inside the main calibre bundle).
For instance, if you want to open a book using it from anther program you would do
Code:
open -a /Applications/calibre.app/Contents/ebook-viewer.app file.epub
Or if you dont want to use open
Code:
/Applications/calibre.app/Contents/ebook-viewer.app/Contents/MacOS/ebook-viewer file.epub
On other OSes, just do:
Code:
ebook-viewer file.epub
@Springbok: the calibre viewer most defintely syncs both reading position and bookmarks across operating systems. In fact, by default when reading epub files it stores this data inside the file, so to sync across OSes all you need to do is sync the EPUB file itself.