Years ago I read a stockholder statement by one of the Big Six where the CEO used the term "units" and there was a footnote to explain "Units are books".
The current wisdom is that the way you become successful in digital publishing is to have a lot of books. Now writers who used to write 1 possibly 2 books a year are planning on writing at least 4.
Here's the truth for me. I may write that fast but I don't think that fast.