As anyone who reads the LOTR books can probably figure out, Tolkien originally concieved of it, and wrote it, as single novel with an overarching plot. However, there was apparently a paper shortage at the time, so Tolkien's publisher decided to break the novel up into three volumes, producing the LOTR trilogy we all know and love.
So, what if the publisher had allowed LOTR to be published as one volume, like Tolkien had originally wanted? One consequince I can see is that the precident for fantasy trilogies isn't established, so we start seeing more single-book fantasy novels and less series. Any other thoughts?