Use an ellipsis (three points) to show that some text is missing, or to trail off in an intriguing manner...

If you're showing that text is missing, leave a space before and after:

"A borrowed book is like a guest in the house ... you cannot leave it carelessly ... you cannot turn down the pages ... and then, some day ... you really ought to return it."

(from William Lyon Phelps, The Pleasure of Books, 1933)

If you're trailing off enigmatically, you don't need to leave a space:

I was beginning to wonder whether William would ever return that book...