Abstract appeared in the abstract book of a conference but the paper is not published, can I publish it elsewhere?
Yes. In my field you don't publish the same but you can present more than twice. Because conferences aren't journals but are seen as moments of discussion and dissemination of ideas and results. 

I would avoid the same wording anyway. This gives idea of laziness at least if one is not sure to have found the perfect wording. 

It might be different in fields where conferences are seen as the major source for results, but still, up to date, your results are not in the wild and they just left a trace as simple abstract.