Talking It Over by Julian Barnes (Trois #1)
Shy, sensible banker Stuart has trouble with women; that is, until a fortuitous singles night, where he meets Gillian, a picture restorer recovering from a destructive affair. Stuart's best friend Oliver is his complete opposite - a language teacher who 'talks like a dictionary', brash and feckless. Soon Stuart and Gillian are married, but it is not long before a tentative friendship between the three evolves into something far different.. Hardcover, 1991 Goodreads: 3.71