"How did Agatha [Christie] develop from the amateur, 'trying things, as one does', into the creator of this dazzlingly accomplished book*? By degrees. By intelligence; by instinct; by confidence; by courage. By trusting her own judgement about what made her writing work. By having a mind uncluttered with received ideas, and an imagination that naturally ran so free she could enjoy the exercise of its restraint. It was a relief, in fact, to trammel it within a genre. Although the structural work of a detective novel was very difficult, there was joy in the discipline. She was ordering her brain like a well-run establishment. Like Lucy Eyelesbarrow in 4.50 from Paddington, she was shining the silver and scrubbing the kitchen table and making a Spanish omelette with the leftover potatoes. She was living in a world where all could be known, all motives uncovered, all ambiguities penetrated. Where mysteries were within her control."
That's from Laura Thompson's Agatha Christie: An English Mystery which I'm much enjoying, and I was amused by the following passage quoted earlier in the book and relating to a period when Agatha had fewer novels under her belt; here she looks back on the early years:
"It was by now just beginning to dawn on me that perhaps I might be a writer by profession. I was not sure of it yet. I still had an idea that writing books was only the natural successor to embroidering sofa-cushions."