Complex sentence: It is the first of a series of eleven, Anderson's most well known work, about a little boy, Billy, and his pony, Blaze, who he receives as a birthday gift in this book.
Split into simpler sentences: It is the first of a series of eleven books and is Anderson's most well known work. The book is about a little boy, Billy, and his pony, Blaze, who he receives as a birthday gift in this book.