It really depends on the image and the
feel I have from the cover or a book - for example if it looks scary I will use scary-looking font.
I wasn't satisfied with fonts pre-installed in my system so I downloaded some from
dafont.com and
1001freefonts.com.
I like to tinker with my covers so it's not "Check all" -> "Generate Cover" type of work but a moment with adjusting size of font or including newlines so that title doesn't start new line in a odd place (like after "a" or first word in a parenthesis)
Some examples:
- Marked
- Croquis
- FreeSans (pre-installed)
- Biko
- Sandrine
But I'm sure it depends on your likings.
I will probably upload some template_settings once I go through what license do the fonts I use have.