I break up my writing this way, going from book to book. It's a weird sort of tick, I guess, this flitting about, the byproduct of a brain that likes variety. If I ever get really popular and sell enough books, I may have to give up my quirk and write one book at a time.
The flitting isn't as random as it appears at first glance. I tend to divide the writing into fours--act one, parts one and two of act two, and act three. I like to think about story structure as being like a three act play and each act has a set purpose. Act one comprises about the first fifth (or so) of the book. It's there to introduce you to the characters and to the setting and to give you the conflict and reason for following the story. Starting stories is always fun for me, although it can be difficult, sometimes, to get the character voices and personalities right at the beginning.
By the end of act one, the main characters have begun to take shape and if I'm lucky, I like having them around. Zoe, the main character of Second Hand, is no exception to this. She's twenty three, bright, funny and somewhat ambitious. Her parents and grandparents have been married multiple times, so it's little wonder she's writing a new blog called Uncoupled, about the breakup of relationships. And it's also small wonder that she has trouble in believing a lasting relationship is in her future.
Zoe has a new boyfriend. Derek is sweet and loyal, a guy's guy with simple tastes who believes in love and relationship. He has trouble keeping up with Zoe's fast-racing thoughts and is both in awe of her and a little taken aback. They aren't really very well matched--something that will play itself out and become more apparent in act two, when conflict gets worse and lives begin to get tangled.
I've got a long ways to go until I write the end onto Second Hand Love. And, because I'll put it aside and pick it up again, it'll take me a year or more to get there. I'm well on my way, though, having finished act one and putting Zoe's story in motion.