FWIW - I think a dual pane interface to two physical libraries would be more useful than virtual library tabs. I appreciate that ain't easy.
If folks are creating lots of VL's then a vertical tabstrip might be better than a horizontal tabbar. It can be put in a split container with a scroll bar when required, and it can use flyouts if the container is too narrow to display the full name, it's hard to do equivalents with a horizontal tabbar. Also monitor aspect ratios are trending wider rather than deeper.
Also - I thought there was a 'display cover full size' feature somewhere in Calibre, but maybe I imagined it - anyway I thought it might be useful to have in the cover grid.
I think I saw a request for a one pixel border to indicate the book is on the current device, IMO it would be hard to discern such a border on a half decent high res monitor. I suggest as ebook reader icon in a corner - let user chose size.
BR