A question I often hear from aspiring writers is whether to lay a project to the side and move on to that Shiny New Idea they can't stop thinking about. My advice is always the same: do whatever the heck you want. Especially if you're unagented/unpublished. Without deadlines to bog you down, why not do what you are most passionate about at the moment? Writing is fun. It doesn't mean that other manuscript is dead. Alas, you are a fiction writer--revive that zombie when you're good and ready.
A multiple book project (in my case, a trilogy) is extremely common. Especially for science fiction or fantasy. But for an unpublished writer? Ugh. Cue my annoying sensible self. How do I query a book that is the first of a series? Do I just write all three even if the first one doesn't snag an agent? I know a lot of you have written a series or at least the first book in a series. How does one cope with these doubtful thoughts of a confessed stickler? Advice, people, advice!