After a brief breather in book three (Lucinda's Secret), the Spiderwick Chronicles heat up with doppelgangers, then dwarves, then a dastardly double-cross, as this five-part series approaches its dramatic end. The Grace kids (9-year-old twins Simon and Jared, and 13-year-old Mallory) might think that things have finally quieted down for them, but the nefarious faerie world has many more surprises in store. In the second chapter, titled, "IN WHICH the Grace twins are triplets," a mysterious and menacing shape-shifter shows up at Mallory's fencing match--and before Simon and Jared can suss out what's up, their sister disappears, presumably kidnapped. Eager to recover Mallory, the two descend into a strange subterranean world beneath a nearby quarry, only to find themselves prisoners and then privy to a wicked (and almost unbelievable) plan. The twins do end up tracking down Mallory, but only in very peculiar circumstances--not the least of which that she's wearing a dress. Author Holly Black once again skillfully manages to weave in plenty of creepy details (including a bloody final chapter) without whitewashing or leaving young readers feeling too creeped out--and she gets able assistance from Tony DeTerlizzi's ever-evocative pen-and-ink drawings (especially in the looming menace of the Mulgarath). Fans of the series will have a hard time waiting for the final installment, titled fittingly, ominously, The Wrath of Mulgarath. (Ages 6 to 10) --P