Before paper became cheap, wax or wooden tablets were used for ephemeral writing.

> I have read a lot of late 18th, 19th and early 20th century books and diaries, and it is plainly clear that writers such as Tolstói, Zweig, Goethe and others developed full books in their mind first, then wrote them from cover to cover in 20-30 days.

I seriously doubt that it was ever common for writers to compose a whole book in their head and then write it down. Maybe some writers with exceptional memories did this. But there's a whole book about how War and Peace was written based on textual evidence that wouldn't exist if it had simply popped out of Tolstoy's head fully formed: https://www.amazon.com/Tolstoy-Genesis-Peace-Kathryn-Feuer/d....

Not war and peace, which was episodic, but smaller novels were thought out in Tolstoy’s mind before being written wholly. He mentions this in his diaries. Zweig mentions the same, too, but of course his novels are generally much shorter than the two Tolstoy’s masterpieces.

Holding long epic poems in your memory alone was once a celebrated skill.