@mrmikel - sorry, you are quite right. Focus after the delete moves to the open tab. You would have to click back in the book browser and then hit Enter, which you may as well double click for. Ok, instead of enter, hit Alt+Page Down
Assignable keys? You can overwrite any of the shortcuts that come with Sigil to your own purposes, I change a number of them to meet my personal needs. The new keyboard shortcut dialog should hopefully make that easier to use, the last version always had me wondering what clear versus reset did etc.
Or by assignable keys do you mean macro combinations? In which case the answer is no discussion I have ever heard mentioned of.