For me Book View is too dangerous to use as an editor. I use only Code View.
Step-by-step to crash:
1. New file
2. In Code View write "ok"
3. Select letter "k".
4. Press Ctrl-I or press "Italic" button from toolbar (to enable italic)
5. Go to Book View (F9 or Book View button).
6. Select italic letter "k" only.
7. Press Ctrl-I or press "Italic" button from toolbar (to disable italic)
8. Try back to Code View (F11 or Code View button).
Last point causes a crash every time.
Sigil 0.9.9 original AND Sigil 0.9.9 with all commits from GitHub.
Windows 7 Pro, Windows 10 Pro