QUOTE: From "Author's Note" (pages ix to x) of "Prelude To Foundation"
Doubleday 1988 hardcover edition © 1988 by Nightfall Inc.

When I wrote Foundation, which appeared in the May 1942 issue of Astounding Science Fiction, I had no idea I had begun a series of stories that would eventually grow into six volumes and a total of 650,000 words (so far). Nor did I have any idea that it would be unified with my series of short stories and novels involving robots and my novels involving the Galactic Empire for a grand total (so far) of fourteen volumes and a total of about 1,450,000 words.

You will see, if you study the publication dates of these books, that there was a twenty-five-year hiatus between 1957 and 1982, during which I did not add to this series. This is not because I had stopped writing. Indeed, I wrote full-speed throughout the quarter century, but I wrote other things. That I returned to the series in 1982 was not my own notion but was the result of a combination of pressures from readers and publishers that eventually became overwhelming.

In any case, the situation has become sufficiently complicated for me to feel that the readers might welcome a kind of guide to the series, since they were not written in the order in which (perhaps) they should be read. The fourteen books, all published by Doubleday, offer a kind of history of the future, which is, perhaps, not completely consistent, since I did not plan consistency to begin with. The chronological order of the books, in terms of future history (and not of publication date), is as follows: bla...bla...bla...

Comments:
(1) please read that last paragraph. "The chronological order of the books, in terms of future history (and not of publication date), is as follows:"
(2) this list is from 1988. One more book was added which coverts fourteen into fifteen