Welcome back, calibre users. It has been a year since calibre 4.0. The two headline features are Highlighting support in the calibre E-book viewer and that calibre has now moved to Python 3.
There has been a lot of work on the calibre E-book viewer. It now supports Highlighting. The highlights can be colors, underlines, strikethrough, etc. and have added notes. All highlights can be both stored in EPUB files for easy sharing and centrally in the calibre library for easy browsing. Additionally, the E-book viewer now supports both vertical and right-to-left text.
calibre has moved to using Python 3. This is because Python 2 was end-of-lifed this year. This should be completely transparent to calibre users, the only caveat being that some third party calibre plugins have not yet been ported to Python 3 and therefore will not work in calibre 5. For status on the various plugin ports, see here. This effort involved porting half-a-million lines of Python code and tens-of-thousands of lines of extension code to Python 3. This would not have been possible without the help of Eli Schwartz and Flaviu Tamas.