I am not sure I follow you here - are you saying that when a book is added to a series, it tends to generate new readers for its predecessors? I would guess that, unless the new book significantly enhances the awareness or prestige of the series, it would have little effect on the sales of earlier episodes. Otherwise, regardless of how good it is in its own right, I doubt it is likely to attract readers who have not already read its predecessors.
It might be of little effect, but I specifically target series. A set of nine will beat out a pair for me. I'm hooked on Brandon Sanderson partly due to the interlinking between his different series.
Exactly. Especially in SF and Fantasy, series present a much more attractive target. You know the author has put in some love & effort into the books, and you know that the effort YOU put into learning the world and your emotional investment into the characters won't be wasted.
You might of course still drop it after the 2nd or 3rd (or even during the 1st) book due to various reasons (you didn't like it, found something better, were busy, family emergency etc..), but the nth+1 book still influenced in that first sale.
Paper book retail has complicated shelving and buying rules which make it a winner's game.
A tiny percentage of the most popular writers gets reprints, new editions, and shelf space. For new writers, the process is more like a sales audition.
And it's a very short audition. If a new title doesn't show serious sales momentum within a few weeks, most copies will be returned, and the odds of a follow-up title from that author go down rapidly.
Virtually all of the n-ologies you see in the bigger stores are already best sellers, and the publishers and the store buyers have both said "Give us more of the same" because previous sales were strong enough.
Some markets - like romance, and sometimes fantasy - have more complicated rules. Store buyers typically buy a consignment from a publisher, and authors can sometimes find themselves sneaking into a consignment without being anyone's first choice.
