Jacqueline may have been hired for her name and for her social relations, but she soon proved her worth. Her choices, suggestions and widespread social relations were of benefit both to the publishing firms and to
Jacqueline herself. In the books she selected for publication, she built on a lifetime of spending time by herself as a reader and left a record of the growth of her mind. Her books are the autobiography she never wrote. Her role as First lady, in the end, was overshadowed by her performance as an editor. However, few knew that she had achieved so much.
Could any native paraphrase the underliend sentence?
A lifetime of spending time on her own reading gave her a basis of experience for her work as an editor. The books she selected form a record of the way she developed mentally.
To take a very crude example: If the first book she selected had been by Ian Fleming, and the last one by Clive Cussler, we would have thought she had a skill for spotting a potential best-seller, but we would know little about her as a person. However, if her own experience of books had enabled her to begin by discovering Virginia Woolf and end with Doris Lessing, we might feel that we understood something about her mental development as a person.