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Heinlein's WALDO / MAGIC INC are good introductions to his work as well. I like GLORY ROAD, but I recall there may be a scene or two that are mildly inappropriate for a younger set.
Regardless, any of his "pulp fiction" early works should be fine... plus, it allows the reader to read the Future History storylines in order without realizing it so when the reader graduates to Time Enough For Love etc etc, everything should magically fall into place on the big global plotline.