Amazing mockups. A few things come to mind:
- I personally don't like to use the cover browser, I'd much prefer a larger book list. I assume this will be user-toggleable the way it currently is, though.
- I didn't notice a search bar. I see you have the advanced search tab thingie on the left side, but does it contain the search bar the way it currently is, or does it contain the advanced search dialog? I'd personally prefer to have the current search bar, because it's just so powerful.
- The book details pane on the right-hand side should still be configurable the way it is today, as in there should be a layout for narrow screens where it is below the book list.
- There should very much be a consistent layout across all platforms, this makes it way easier to help users, and it looks nicer. That said, remember that Linux and Windows will have the menu bar below the title bar. No biggie, just something to remember.
You mentioned making it easier for the casual user to use the program. I'll just note that many of Calibre's advanced features are designed for what you call power users. This has historical reasons, and those features should very much still be easily accessible- you don't want to dumb it down, just make it easier to use. That said, I think your design does that.
Edited to add: I just noticed this: My workflow relies somewhat heavily on view restrictions. Those should, I think, be readily available somewhere in there.