About the dragged files disappearing in the Book Browser panel, you're running under KDE4, this is a long standing bug in Sigil <-> KDE4; however there's a workaround that seems to work.
In a terminal emulator (e.g. Konsole), unset the QT_PLUGIN_PATH:
Code:
unset QT_PLUGIN_PATH
then launch Sigil (from the same terminal):
Code:
/opt/sigil/sigil.sh
or however you usually launch it. Now dragging/dropping files in the Book Browser should work as expected.
As an alternative, you can edit the content.opf file, reordering the entries in the <spine> section manually then saving/opening the file anew should do the trick (it's advisable to work on a copy of the file so that if anything goes south, you can reuse the original file).