I think Iain M. Banks did it best with books set in the same universe but often making out how a book participated in that universe — e.g., Inversions — was rather obscure. That is, he never depended upon it.

It was my understanding that publishers liked series, dependable follow-on sales and all that, and that it was hard(er) for new authors to break into/onto the scene unless they had a series behind a first novel.

Finally, the pressure to make thing into trilogies, and trilogies into series really spoils the possibility of a right-sized pair of novels.

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John Laudun

John Laudun studies how information flows across social networks online and off. For more, see http://johnlaudun.net/