When should a book's cover mention that it's the second one in a series?S

Do authors and publishers have an obligation to make it clear to readers up front that a newly published book is actually one of the middle books in an ongoing series?

Or should they do that only if, say, a book resolves a cliffhanger from the previous book and ends with another cliffhanger, or has other clear indications of being a serialized narrative? After all, some books may have ties with other books set in the same world, but may not be as simple as "part 2 of a trilogy."

This question seems to come up a fair bit — I've seen several new science fiction and fantasy titles in the last couple years whose covers didn't make it clear whether they were fully stand-alone works. And author Tim Gebhart brings the question up again, when reviewing the new novel by Kris Saknussemm, Enigmatic Pilot: A Tall Tale Too True. (Disclaimer: I haven't read any of Saknussemm's work, although I've heard great things, so I'm not qualified to have an opinion on this particular case.)

Gebhart reviewed the book for Blogcritics.org, and it was republished in the Seattle Post-Intelligencer, where I saw it. And although Gebhart is a fan of Saknussemm's writing, he takes issue with the fact that you won't fully appreciate Enigmatic Pilot unless you've read the first book in the series, Zanesville:

Enigmatic Pilot is the new installment in that series but while Saknussemm's writing remains strong, the book suffers not only from being an installment in a series but from the fact that those unfamiliar with Zanesville may not realize it is part of a series...

While readers may not need a detailed road map, to leave them without any of the background that informs the story or that Enigmatic Pilot is part of a series is to leave them feeling as if they have been on several detours to nowhere. Yet the book design and marketing don't even hint that Zanesville might give readers insight into some of the symbolism and plot threads in this book. In fact, Saknussemm's bio on the Del Rey website makes no mention of Enigmatic Pilot even though it does say Zanesville is the first in The Lodemania Testament series.

In his review, Gebhart goes into a lot of detail about all the signs that the two books are part of a series. And he lists rather a lot of stuff in the new book that will either fail to make sense, or feel pointless to the reader, if he or she is unaware of the earlier book. (Like I said, I haven't read either book, so I have no opinion of my own here.)

In any case, it seems like there's a valid issue here, as well as a valid question — where do you draw the line? How clear-cut do the links between two books need to be, before publishers or authors have an obligation to market them as part of a series, rather than stand-alone works? [Seattle Post-Intelligencer]