Mary Pope Osborne 
Magic Tree House 3: Secret of the Pyramid 

Eight-year-old Jack and his little sister, Annie, are playing in the woods during their summer holiday, when they find a mysterious tree house full of books. But these are no ordinary books . . . And this is no ordinary tree house . . .
When they»re whisked away to ancient Egypt in the tree house, Jack and Annie learn how to read hieroglyphics with the help the ghostly Queen of the Nile.
But secret codes aren»t the only puzzle they have to solve. How do they get out of the pyramid full of mummies? And there»s still the problem of who all these magical books belong to. Who is this mysterious M?