If you’re in the process of marketing a book, I wholeheartedly recommend that you get well into another project prior to sending out your completed manuscript.
I got lucky with timing, finishing the final draft of my novel in early October, which meant that when I participated in NaNoWriMo in November, I ended the year with one finished novel and one rough draft under my belt. Once I recovered from NaNo, I started researching agents. All of the times which would’ve been empty spots in my writing life (the month or two break most of us take between finishing a draft and starting the rewrites, the waiting game with the agencies, etc.) were filled up, because I could switch back and forth between tasks for one manuscript and tasks for the other.
Now that I’m well into the querying process and doing a lot of waiting and not much else for my finished novel, I’m so grateful that I have another book ready to be worked on. As antsy as I am with a project to work on, I can just imagine how much worse it would be if my writing life, right now, consisted purely of sending out letters and samples and then waiting for replies.
Aside from providing a welcome distraction and being an efficient use of time which would otherwise be spent chewing your own face off from the inside, having something else to work on is also a good mood booster when you get a rejection. At least, it works that way for me. If I work out a problem with my rough draft, write a new scene I really like, or come across something awesome when I’m fact checking my details, it takes a little of the sting out of getting a rejection. Even if it’s a bad writing day and I get a rejection, I can tell myself, “But see, you’re a real, professional writer. You’re already working on a new book, the way professional writers are supposed to. You’re just waiting for your break, and getting work done in the meantime. See how awesome you are?”
Anything that boosts your confidence and makes you feel good about yourself, that’s what you want to do while you’re marketing your book. So write, write well, and write something that makes you happy.