Temporary Quarters — Barbara Hambly

A novelette of 14,000 words in the Antryg Windrose series. Joanna's cousin Robyn is positive her newly-wed husband has been possessed by some evil spirit, who is actually much nicer and more polite than the original... but given the amount of power it takes to possess, Antryg is pretty sure there's something dangerous afoot. He and Joanna go to investigate in one of those lovely resort towns on the Central Coast of California, which seems to consist entirely of occult and New Age businesses, hot tubs, and antique malls.