In my opinion, the book has this title because the "hiding place" that the ten Boom family provides is the main reason for the book being written.
The book describes the ways in which the ten Boom family reacted to the Holocaust. It talks about what they did to hide Jews from the Nazis (thus the title) and it talks about what sorts of persecution they suffered when they were caught.
The title also refers to a line from Psalm 119, talking about God as a person's hiding place from the evils of the world. This makes sense because the book is also about the ten Boom family's faith and how it was challenged and strengthened by their ordeal.