Nearly 500 books. Science fiction novels, chemistry textbooks, murder mysteries, histories, guides to Shakespeare and the Bible, even a collection of limericks. Asimov wrote across almost every shelf in the library, and people still ask how one man produced that much. The answer turns out to be less mysterious than the legend suggests.

The Direct Answer
He wrote every day and kept composing in his head even when he was nowhere near the desk.
Asimov rarely outlined and ran many projects at once, so the pages stacked up across every category, per his memoir I. Asimov.

Always Writing, Even Away From the Desk

Asimov didn't outline in any heavy way. He'd start with a situation, sometimes only an ending he wanted to reach, and type his way toward it. The reason that worked is that his mind never really clocked off. While he ate, walked, or drifted toward sleep, some part of him was quietly working out the next scene or the next argument. By the time he sat down, the writing felt closer to transcription than invention.

Everything is, in a sense, already written. I can just sit down and type it all out.

Isaac Asimov, I. Asimov: A Memoir

That's the whole secret behind the missing outlines. The structure had usually taken shape in his head during the hours away from the keys, so the blank page held no terror for him. He treated writing as the easy final step of a process that had been running in the background all day. See the full Isaac Asimov writing routine for the schedule that gave all that background thinking somewhere to land.

Range Kept Him Moving

Plenty of prolific writers stay in one lane for a lifetime. Asimov refused to. He'd wrap a science fiction novel and turn straight to a book about chemistry, then a guide to the plays of Shakespeare, then a joke collection. His books eventually landed in nine of the ten major categories of the Dewey Decimal System, missing only philosophy. The constant switching kept him fresh, and it solved the biggest killer of output.

When one project stalled, he simply moved to another and came back to the first one later, so he was almost never stuck staring at a blank page with nothing to do. He also loved the work for its own sake, and money wasn't the engine. When an editor once cautioned him about writing so much, Asimov reckoned the extra books gave him ten times the pleasure, and pleasure was the entire point. A man who enjoys the task that much needs a reason to stop, not a reason to start.

Writer's block barely touched him, and the switching was the reason. If the science fiction wouldn't come one morning, he'd set it aside and work on a textbook chapter or an essay until the jam quietly cleared. He never handed a single stuck project the power to shut down his whole day. There was always another page he could be filling somewhere else on the desk.

Put it all together and the mystery thins out fast. A writer who works every single day, keeps composing between sessions, holds a dozen projects within arm's reach, and would honestly rather be writing than doing almost anything else is going to leave a very large shelf behind. Asimov once said he wrote for the same reason he breathed, and by the end of his life the two habits really did look about equally automatic.

What You Can Steal