Sithe in the City

This is the 3rd book in a series "Sithe in the City" It begins with "Amy and the Ankylosaurus" and continues in "Helen and the Evil Elves of the Elder Gods" The series tells the story of Magnus, a palenotologist and physicist who builds a device that allows crossing from one quantum branching universe to another, in order to impress his ex-girlfriend Amy, a curator at the museum. First they accidentally run afoul of dinosaurs. In the second book Helen wants an elf boyfriend, or something like that, and blackmails Magnus into another universe crossing. In this book one of the Evil Elves of the Elder Gods takes a liking to Magnus and comes back to New York City with them. This is a problem for Amy who is now Magnus's girlfriend again. The girls go shoe shopping and thwart a mugging, all while looking fabulous. And Magnus tries to figure out who to sell his dinosaur clones to.