POLAR is a powerful offline browser for Mac, Windows, and Linux for managing all your web content, books, and notes. Polar keeps all your content in one place, supports tagging, annotation, highlighting and keeps track of your reading progress. POLAR allows you to keep all your documents, annotations, and flashcards in once place.
I used to use Polar Bookshelf before, when it was still open source. It was very easy to use, however, I don't know what happened now, the open source version was gone, and the new Polar 2 and Polar 3, there are no update of it any more, and I am really want to know what happened behind it.