@domee - I'll have to think about that, I guess it might be possible. Though it has sparked another idea instead - the ability to save as part of the state the visibility of the tag browser panel and the book details panel (and cover browser too I guess). So with a single keypress to switch to your view you could effectively change the whole layout of your GUI?
Trying to do that and trying to save widths of panels would get a bit too complex I think. My preference would be for the ability to specify panels to be visible/hidden. Does anyone else have any thoughts on this?