The author claims poetic biographies are best for recitation, but then writes the Preface in prose. Isn't this a contradiction? Why didn't he write the Preface as a poem?

This is an astute observation. In Chapter 0, the author argues that for the specific purpose of devotional study, recitation, Parayans (hundreds of readings), or Saptahs (weekly recitations), detailed poetic biographies are most suitable because prose is difficult to memorize. However, the Preface itself serves a different function. It is an introduction meant to explain the context, principles, and importance of the main work. It is not intended for the kind of ritualistic recitation for which poetry is recommended. Therefore, writing the Preface in prose while advocating for poetry for the main biography is not a contradiction but a choice based on the distinct purpose of each section of the book.