A button fastens and closes, so dream books read it as keeping things together and presentable. A lost button is taken as a small loose end; undoing one, as relaxing or opening up.
Does the traditional reading fit your dream, or did it mean something else to you?
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.