Did St Teresa want to write this book on prayer? No. She felt great repugnance both by a sense of her lack of ability and her constant headaches. But she trusted in God to help her. If she was being commanded by the Church to write this book, she would trust God to teach her what to say. She began on the Solemnity of the Holy Trinity.
This book is written after St Teresa had vast experience of prayer. It is her masterpiece. A woman writing primarily for other women. She prays that her work will be of profit to others. Above all, she hopes that her effort will lead to Christ being loved and praised by even just one other person.