The title is like a glimpse at what is going to happen in the book. For example, if a book iks called 2012, it almost certainly going to be set in 2012. I think that in the novel that I am curently reading is called six impossible things. i think that this is a very fitting title. Another title could be the six challenges, or the impossible achievements. I am afraid that, to my knowledge, it is not in a series.