I don't know about paid, but free advertising is available right here at Mobileread - as long as an author or publisher is willing to follow the Posting Guidelines, which of course means reading them, a challenging concept for some individuals!
It absolutely boggles my mind that someone will not read our Posting Guidelines before using our site before engaging in a promotional scheme. (Would someone sit down uninvited at someone's house and not observe decent table manners? -- Well, if you're Uncle Buck, then OK.)
Anyway, we reach a very large audience of potential buyers, although the breadth of promotion may not be as broad as some authors and publishers would like.
