You've probably heard people say that if you're working on a book, or another creative project, you should put everything you have into it and focus on one thing at a time so as not to get burnt out.
This, I am certain now, only works for some people. If you're like me, you're restless in your projects. If you're excited about more than one project at a time, work on all your ideas. You don't have to be equally far along on each one.
There are different reasons for doing this. Some people get bored writing one book and go to work on a short story or even another novel length work to get them going. Other people consciously set one aside for a while after they've finished a first draft so that they can work on something else. This allows them to let the first manuscript cool so that they can come back to it with different eyes.
As for myself, I enjoy working on more than one book at a time. My books and projects are all so different in tone that I like visiting those tones and moods when it suits me. Also, as different as all these projects are, I wade around in the same few themes, and I feel like my books have lessons to offer one another, if you will. A theme that might be only discussed in one book might actually turn into an allegorical story structure in a different book. I also might write a straightforward essay on this already-used theme for The Burning Block (an upcoming magazine from Emergent Hermit). I work on more than one thing because, I get bored, because I want to come back to a work with fresh eyes later and because I just enjoy that freedom.
I think everyone has to do what works for them. I certainly don't disparage anyone for working on one project at a time--there are certainly benefits. But don't let anyone tell you that's how you should do it. Why should you? Do you think a publisher is going to sniff your manuscript and say ‘I think this suffers from the impurity that comes with multi-tasking.' No, they'll either like it or they won't. What's the worst that could happen? Someone will judge you? If that's your most primary, undefeatable concern, then you might as well lie. Tell them you're only working on one project if that makes you feel better.
Then, you might ask, how often do I switch between them? To some people, switching projects this often would be tantamount to an artistic identity crisis. If you think it would be too hectic or whimsical to sit down each day and figure out what you feel like working on, you might try a program. Work on one project on certain days and another project on other days. Maybe you'd like to work on one project for a whole week and switch it out the next week, the next month, even the next day.
Personally, I don't have a problem working on more than one book or project in a day. I don't always have time to do this, but it's certainly not an inhibition anymore. But, as with everything, figure out what works for you. Don't be afraid to try different things. One thing it's easy to forget as an artist is that the final product is what matters. Don't get so wrapped up in your process that you think the work is suffering simply because you're not having an easy time of it. You'll get there with time and patience. Allow yourself room to breathe and enjoy yourself when you work.