I can relate to this from the Chronicle of Higher Education:
“And then there’s the reference manual on amphibians and reptiles that first crossed her desk in 1994, during an earlier stretch of her career at the Kentucky press. The author in that case, Ms. Salisbury wrote, had spent more than 30 years collecting data for the book. But every time publication seemed imminent, the author would discover a new data point — say, a new span of territory that a lizard might inhabit in the state.”
At some point you just have to declare a book done.