Getting book pages in same level like getting child pages
If book module is enabled, it also creates a block called "Book navigation". You can display the block in any of the "Region" defined in your theme (e.g. "Sidebar first" in Bartik theme), by visiting http://your-site-url/admin/structure/block.

The block will be displayed(in the selected region) whenever you open the book page. And by default it will show the the book pages at the same level(sibling) pages.