What two secrets are prominent to the development of the plot, and how does each secret impact the person/people keeping it?

One example of secrets is Rosa and Hans' protection of Max. In hiding Max, Rosa and Hans put their family at risk. For Max, it creates feelings of guilt.... worry that his presence might do them harm.

Liesel's identity as a book thief isn't quite as worrisome. She is drawn to books, and she is drawn to stealing them. For Ilsa, Liesel's actions become a basis for building trust. Liesel's theft at the bonfire, however, puts her family in danger.