From Time to Time by Jack Finney

In Finney's wonderful cult classic Time and Again (1970), Manhattan adman Simon Morley joined a secret government time-travel project, transported himself back to the New York City of 1882, fell in love and decided to remain in the past. This entertaining sequel, which traces Simon's attempts to alter a course of events in 1912 and thereby prevent WWI, lacks the magic and urgency of its predecessor but is diverting nonetheless. Bidding goodbye to his 19th-century wife, Simon first revisits the late 20th century, where remnants of the "Project" propose another experiment to redirect history.