Ask HN: Focus on a single side project?

Context: I have a successful SaaS side project that I have built and launched, and it's doing great. I spend very little time on my first SaaS (~4 hours/week).

I'm in the middle of launching a new SaaS. The new SaaS is my "side project". It's a for-profit, non-trivial service. However, I often get an urge to have a second project going such as a book. Do you fight these urges or give in?