Storerooms keep what is not needed now but not thrown away. Dream books read them as memories, skills or feelings set aside for later, and what you find inside as something ready to come back into use.
When this appeared in your dream, what were you feeling?
My Cosmic Code has a private dream journal. Write a dream down and the app picks out the symbols in it, shows what dream books say about them and, over time, which words recur across your entries. The journal is stored on your device only.