The Wells Bequest by Polly Shulman 260 pages
In this companion to The Grimm Legacy two teenagers use H.G. Wells famous time machine to race through time and stop a dangerous enemy. The book starts with Leo meeting a tiny future him and unknown girl in his bedroom. He sees her for the second time in a library that circulates many items but not very many books. He gets a job there and gains an enemy which starts their quest to go back in time.