From the book: Christie, Agatha. An Autobiography . William Morrow Paperbacks. Kindle Edition.
But we were happy there, and I planned to take a course of shorthand and book-keeping which would occupy my days. So it was goodbye to Ashfield and the start of my new life, my married life.
I enjoyed learning shorthand and book-keeping. I was humiliated by the ease with which little girls of fourteen and fifteen progressed in shorthand; at book-keeping, however, I could hold my own, and it was fun.
[As I recall, I don't believe she never did any office work, I think WW One started and she worked as a 'nurse', got sick so then worked in a type of pharmacy, which explains why she uses poison in many of her novels.]
[later in the book she was looking for an assistant:] and I could then have a secretary shorthand-typist for some hours at my beck and call. Perhaps I should be able to dictate my literary works. It seemed a good idea. I put an advertisement in the paper, asking for someone who would look after a child of five, shortly to go to school, and act as secretary shorthand-typist–I added ‘Scottish preferred’.
[one person she interviewed:] She knew shorthand and typing, but had not had much experience recently in shorthand.
[the person she hired:] Although she had taken a shorthand-typing course she had never had much practice in it, and indeed had tried to refresh her skills by taking down sermons. She was terrified that I would rush along at a terrific pace–but nobody could have found any difficulty in taking down what I was saying. They could have written it in longhand.
[later she said she preferred to type or handwrite her novels as she didn't like the sound of her own voice. However, she tried a Dictaphone when she broke her wrist:] The disadvantage of a dictaphone or tape recorder, however, is that it encourages you to be much too verbose.
Those were the only searches I could find in "shorthand" and I finished it last year, so I can't remember if there was anything more.