In your latest blog post "Talking about books within a series" you mention how nothing scares editors more than mentioning series. What about books that are expected to form part of a series, like cozies? Do you mention anything about series potential? My finger is perched on the delete button in anticipation...
No no. Don't delete yet.
Most contracts for traditional mysteries are multiple-book contracts. That is, you submit the first one, editors offer to buy two or more, most often with the same characters, in the same setting etc.
A good example is Loretta Sue Ross's Auctionblock series.
The first was Death and the Red-Headed Woman, and then three more followed.
When I pitched this I did NOT say "it's a six book, ten book, six hundred book series." This, despite the fact that LSR would have written six hundred books without a single complaint if I'd told her that was the plan.
What I did say is "this is a book that lends itself to a series."
Particularly with traditionals, readers come to hang out with the characters, they don't come for the plot. This is true of most series actually. I don't care what the plot of the next Reacher novel is; I just want Jack. I don't care what Harry Bosch is solving; I just want to hang out with him while he does it.
This was the interesting fresh twist that Dick Wolfe provided in his ground breaking Law & Order series: it wasn't about the characters, it was ALL about what the crime was. We came to watch stories ripped from the headlines, not to see Chris Noth looking ferociously sexy. (oh wait...)
To answer your question: the distinction is specificity. "This book is the first of six" implies all six are needed to complete a narrative arc. "This book lends itself to a series" suggests it can function as a standalone.