I don’t know if other writers work this way, but I have so far found that working concurrently on two radically different book projects suits me rather well. As soon as the non-fiction project goes out the door for edits or whatever, I get my head back into the other fictional project. In the process, I ‘forget’ about the former and concentrate on the latter. And I have a fresh eye when the former eventually comes back to me. A slightly schizophrenic way of working maybe, but I’m finding it wholly agreeable.
The plan is to have Project T’s premise, plot, pitch, synopsis and first three chapters written and in a fit state to send out within the next three months. The first two points on that list are already finished (subject to many unexpected, interesting changes no doubt).