How do writers determine when their book is complete and ready for publication? Is there a specific process or feeling that guides this decision?

Writers worry about their book. They want the best book possible to go out to publishers.

They often use critique groups, beta readers and editors and work through the entire checklist of problems.

When I felt there was nothing more of value that I could do, I moved on to the next book.

Note: writers often do their research for the next book while editing the last book. They often have many books in work at various stages.

This is because research, writing and editing take different mental and emotional skills. Emotions especially are hard to capture so when you're feeling a certain emotion you pay attention and get that scene down.

I worked with a writer that wasn't sure how his story should end. I realized he cared about it deeply about the person it was about and it involved for him a moral quandary. Some stories hurt too much to write, often they have to be put off until you reach the emotional distance needed to write it.