I've been thinking about why I like some authors' books so much that I will buy (and read) all of a particular series, and others' books which are singletons I will read the one, maybe, but not necessarily any more.

This is apart, of course, from the usual requirements of good narrative, realistic dialogue, tight plotting, and competent editing.

What keeps me coming back for more is the dualism of a personal (and perhaps historical also) storyline that reaches across several volumes, with each book carrying its own plot, embedded in the major storyline, complete within the single publication, while the arc of the series continues on.

The Lord of the Rings is a perfect example--the journey of the Ring and its final destination the overall story; within each volume the various stories, such as the Breaking of the Fellowship. The great theme the contest of good and evil. I couldn't have stopped midway if I had wanted to.

I remember reading Julius Caesar's Gallic Wars (in Latin) in high school. He starts out by saying all of Gaul is divided into three parts (Gallia est omnis divisa in partes tres). Well, you can hardly stop with just one part, now, can you? Even if he does go on and on and on, seemingly ad infinitum.

Recently I've read more loosely structured series, often based upon the meeting/romance/breakup of old relationship, etc. themes. In my opinion, a much weaker place to structure a series, but it can be engaging even so.

Stand alone books may or may not get me to read another by the same author, depending on their quality. Series books, even when weaker individually, even when based on tenuous and well-worn plot lines, tend to keep me buying and reading the next one.

The ones where I am hooked are when the overall plot is complex and creative and a different aspect of what may be familiar ground.