So, this is a tip for everyone who is like me, as in you have a constant flow of ideas for books, but you have found out that you lose interest quickly if you have more than two projects at one time.
Every time you have an idea that you think deserves a story, don't say to yourself "I'll do it once I am ready with my current project" but instead write a short story about it. Short stories are easily written in one evening so they don't take too much time from your main project, but there's still tremendous potential to tell great stories. And who knows, once you've done this enough you could technically collect all of your stories in to one book.
An example, I am writing a fantasy novel, and one night I was playing half life 2 (for umpteenth time I might add) and when fighting the combine, I thought to myself "damn, wouldn't it be interesting to read a story that is set in a similar universe but it tells the story from the perspective of "the bad guy" (as in you generic combine grunt)
So what I did was that I wrote short story, "S&D" (short for "search and destroy") which tells a story of a combine(ish) soldier and his squad conducting a S&D mission in the radioactive canals of the "city 77"
Source: reddit post