It depends on book type. Each chapter in a book has all individuals in that generation and they are sorted by alphabetical order. If you get only your ancestors or descendants, the target will be the only person in the first chapter.If you get both ancestors and descendants (such as book to have target's siblings or cousins), those in the same generation as the target will be in the first (or root) generation chapter.
P.S.: I have book script now rewritten using internal scripting (some testing remains). It is 3-4 times faster. It will be in next version.