In regards to writing a series, when pitching the first of the collection, must it always be completely stand-alone, as in, can quite a few mysteries and questions be left unanswered for to be spread throughout the rest of the books? It's just I have been assuming that series have to be pitched as though the first book is a single novel, so that if only one book is agreed upon it still makes sense if it isn't continued.
Lots of publishers have blogs on their sites, and I've been reading loads of them while researching. Good news, series potential comes up all the time. Bad news, potential. I don't think I've come across any that don't expect each book to be anything but a standalone. Not saying I'm completely right, you'd have to check in your own genre. Hope that helps.
The first novel should be standalone, in that it is a satisfying read for the reader. No cliff hangers. Yes, there can be issues that remain unresolved, but the main conflict should have been addressed and dealt with (successfully or unsuccessfully).
Unless you are going straight to on-line publishing where there are very strong trends towards the first in a series being more like the "hook" scene in a movie and are meant just to draw the reader into the character's life (that continues on throughout the series).