Your writing - your interests
One problem I often face is that I get bored half - way through. But I've found a useful tip - decide that the chapter you are writing will be the best chapter in the book. Fill it with everything you'd want in a book/chapter yourself, fill it with all your best, favorite tricks. Finish the chapter. And write the next, deciding that it is going to be even better than the last one. Next chapter - even better. See the pattern? It works very well, for me at least.