I haven't heard of the author or of the book you are referring to, but since it seems to be a book about finding husbands for women who strongly desire to get married, the description of the author as "very married" emphasizes the fact that this author is indeed married herself. Furthermore, it implies that she might be somewhat of an authority on the subject matter.
"...feeds into the same old-maid prejudice": First of all, an old maid is an unflattering, old-fashioned term for unmarried women, especially those who are above what is considered by a society to be the "marrying age". "...feeds into" means that some of the ideas in this book are the same as old-fashioned thinking on the matter of unmarried older women.
Do these explanations help?