The first thing Dahlia did as soon as she returned home was to hide the book. There was no risking it. She had suffered to get this book and her efforts would not be in vain.
As much as Angelica and Sarafina helped her get it, she still couldn't entirely trust them in case one of them turns dark. So she didn't hide the book in her bag but under the bed where none of them would expect it.
If any of them came after the book for reasons best known to them - or simply part of her imagination - they would probably think it would be in her belonging. So yeah, the bed was a good hiding spot.
Once done, Dahlia returned to the field, only to discover that the match was ending. She found the two girls, Angelica and Sarafina in the crowd already catching up with their fellow dancers who were probably asking where they'd been but Dahlia trusted they could handle that.
