How could Asimov write so much?

From Yours, Isaac Asimov, pg 12

"Rightly or wrongly, I rarely wade through primary material. My references are generally a variety of encyclopedias, dictionaries, textbooks, etc.,-in other words, predigested stuff and secondary material.

This would hurt myconscience if I ever pretended to be contrinuting anything to scholarship, but I don't. I cheerfully admit that I never present anything new. What I have to sell is arrangement and style.

I was once stopped by someone when I was writing one of my books on words because I had a Webster's Unabridged open before me. He said, "Why, you're just getting your material out of Webster's."

And I said, "That's right. Here's Webster's and here's what I've written so far. Do you want to continue?""