When you download a font you will often find a 'read-me' document where the creator of the font has a few words to say but other than their simple requests (i.e. let them know if/how you use the font) it seems to me that these fonts are truly free for commercial use. Which is important because the last thing you want is for your book to get popular and then get sued by a font maker or font website because you didn't have the rights to use their font in that capacity.
Here are a few of the fonts that really struck me as being awesome enough to be on the cover of a book. The story is an urban fantasy with vampires so I tried to find fonts that worked with that idea.