What is the meaning of a "motif"? What does a dash mean over a note? What is the best book of instruction for a beginner, a child of ten?

A motif is the germ of a theme. A theme may be composed of reiterations of a motif, or by grouping several motifs together; it may also combine both modes of procedure. The most glorious exemplification of construction by reiteration of a motif you will find in the opening theme of Beethoven's Fifth Symphony. A dash over a note enjoins the player to hold that note with the finger until it has received its full value. The best "instruction book" for a child is a good teacher who uses no instruction book, but imparts his knowledge to the child from out of his own inner consciousness.