In the second of the "Merlin Missions"--hardback additions to Mary Pope Osborne's Magic Tree House series--plucky heroes Jack and Annie must once again must travel back in time to rescue Camelot from looming danger. One wonders why Merlin can't handle this sort of thing himself, but then of course we wouldn't get a chance to see Jack and Annie have another seat-of-the-pants adventure, getting mixed up with shape-shifting magic, armies of birds, a puzzling gem of power, and all sorts of other trouble. With the help of their old pal Teddy (Morgan le Fay's apprentice, last seen in dog form in four earlier Tree House stories), the two "Master Librarians and Magicians of Everyday Magic" must solve the mystery behind a castle full of ghosts and a menacing army of ravens. Not surprisingly, half the trouble comes in unraveling Merlin's riddles and helping Teddy use his rhyming magic correctly. Osborne doesn't challenge readers overmuch (including the constant restatement of plot elements, perhaps worried that kids might otherwise forget or lose interest) and many parts of the story barely convince (like Teddy's "period" dialogue, e.g., "'Tis cool indeed"), but fans of the Magic Tree House will no doubt love another installment. (Ages 6 to 9) --Paul Hughes