The Faithful Wife, by Sigrid Undset

This is one of Sigrid Undset's contemporary novels.  It deals with the difficulties of married life, particularly infidelity.  Nathalie and Sigurd have been happily married for a long time.  Nathalie, except for her inability to have a child, seems to have it all:  a husband she loves and gets along with, a job she enjoys and is good at.  Sigurd, on the other hand, has felt insecure since he had to give up his engineering practice due to some business problem with his partner, and has been an employee ever since then, not earning as much as he would like.  He starts an affair with Anne, a much younger woman who looks up to him, and Nathalie finds out about it when Anne gets pregnant.  It's a very realistic story, and Undset shows her talent for characterization by making all the characters quite complex so that you see both the good and the bad in each one.  These are three fundamentally decent people caught up in a dilemma.  Great read.