I just implemented a quick toolbar filter for Bookcase. It was rather quick and painless, and I was actually surprised at how fast the view filter is. I was also suprised at how useful I immediately found it.

It just filters all the items by a string, checking every field for each book. I plan to add an advanced filter dialog to allow multiple filter rules and regexps. But the nuts and bolts are there and this will be in version 0.6. This is rather fun! KDE makes it amazingly easy. I ended up copying a toolbar action for a combobox from Konqueror since I want to retain compatability with KDE 3.0.x. Otherwise, I would just use the new KWidgetAction object from KDE 3.1.