Not Just Friends by Shirley P. Glass: A Therapist's Review and Key Takeaways for Couples in 2026

"We're just friends." Few sentences come up more often in my counselling room, and few do more damage when they turn out not to be true. Many of the affairs I help couples recover from did not start with a plan to cheat. They began with a colleague, an old flame on social media or a sympathetic confidant, and slid across a line neither person noticed until it was far behind them. Shirley P. Glass, the US psychologist and researcher the New York Times called "the godmother of infidelity research", built this book around that pattern. Drawing on decades of studies and her own clinical caseload, she shows how good people in good marriages drift into emotional and sexual betrayal, and how couples can recover. My quick verdict: this is the best book I know for understanding emotional affairs, and one of the most useful for couples who want a research-based path to rebuilding trust.

Its main benefits are clarity and evidence. Glass treats discovery as a genuine trauma for the betrayed partner, explains why the involved partner must become part of the healing, and gives both of them a shared language for what went wrong. In sessions I use her image of "walls and windows" all the time: in an affair, a wall goes up against the partner and a window opens to someone else, and recovery means putting the window back between the couple and building a solid wall against the affair partner. Couples grasp that picture quickly, and it turns a vague sense of betrayal into concrete changes they can make. I hold a traditional view that faithfulness matters, and Glass, who describes her own long marriage, writes from the same conviction.

The paperback runs to 448 pages (ISBN-13 978-0743225502, Atria, published February 2004). Glass wrote it with Jean Coppock Staeheli, credited by the publisher. It is organised in three parts, moving from how affairs begin, through the crisis of discovery, to repair, and closes with an Afterword that sums up strategies for protecting a relationship against future betrayal.

The early chapters tackle what Glass calls the new crisis of infidelity: affairs that grow out of genuine friendships at work, online or in social circles. She challenges common myths, such as the belief that affairs only happen in unhappy marriages or that being a loving partner is enough protection, and describes the warning signs of a friendship slipping into an emotional affair, especially secrecy and emotional intimacy that exceeds what is shared at home.

The middle section addresses discovery. Glass frames the betrayed partner's obsessive thinking, flashbacks and sleeplessness as normal trauma reactions, and explains the involved partner's divided loyalties. She sets out how to re-establish safety and goodwill in the first weeks, including ending contact with the affair partner. I find this framing calms many betrayed partners who fear they are "going crazy".

The final section is about healing. Glass argues that couples recover best when the full story is told in an atmosphere of empathy rather than interrogation, and that exploring the individual, relational and social vulnerabilities behind the affair is vital. She covers forgiveness, handling an affair partner who keeps making contact, what to tell children, and how to judge whether a marriage can be saved. The walls and windows model runs throughout.

This research-led classic stands out for explaining how affairs actually begin:

• Strengths: Glass's greatest strength is credibility. She grounds her guidance in her own studies and clinical work rather than opinion, and reader reviews repeatedly describe the book as informative, straight-talking and validating. Betrayed partners often say it finally explained why they feel the way they do. In my practice, the walls and windows model helps couples agree on concrete boundary changes instead of arguing about intentions.
• Weaknesses: The book dates from 2004, so it predates smartphones and social media as we know them, and some reader reviews note that it feels slightly dated and American in its examples. A few readers find the statistics heavy going, and some of the survey samples are small. Practical exercises exist but are spread through a long book, which is why the practicality score sits below the therapist rating.
• Practical Applications: Couples can draw their own walls and windows: each partner sketches where openness and secrecy sat during the affair and where they need to sit now. The involved partner can agree specific transparency steps, such as ending contact and sharing information freely, while the betrayed partner notes which questions still need answers. Reading a section each and then discussing it calmly, without interrogation, mirrors the empathic storytelling Glass recommends.
• Current Relevance: Relevance in 2026 is arguably greater than when it was written. Messaging apps, social media and remote working have multiplied the "just friends" connections Glass warned about. Her core insight, that secret emotional intimacy is the first warning sign, applies directly to the online and workplace affairs I now see most often in clinic.

Choose Glass when you need to understand how an emotional, workplace or online affair developed and which boundaries will protect your relationship from now on. Choose Infidelity: A Survival Guide by Rhys Lewis (my own book) when discovery is recent and you need a practical crisis roadmap for the first days and weeks, whether you rebuild or part. Choose After the Affair by Janis Abrahms Spring when you have decided to try again and want a structured, stage-by-stage plan for earning back trust and intimacy. The Survival Guide steadies the crisis; Glass explains how it happened; Spring shows how to rebuild.

When choosing an infidelity book, start with what you most need right now. In the first raw weeks, a practical crisis guide on safety, emotional regulation and short-term decisions usually helps most. If you are struggling to understand how a friendship became an affair, look for an author who explains emotional infidelity with real research rather than moral judgement. Once you are rebuilding, favour clinician-written books with concrete steps for both partners. Glass fits the understanding and boundary-setting stage, and also suits couples focused on prevention.

From a therapeutic perspective Not Just Friends supports two essential pieces of affair recovery: understanding how the betrayal happened, and rebuilding the boundaries that keep a relationship safe. Glass's trauma-informed approach helps betrayed partners make sense of overwhelming reactions, while her emphasis on honest, empathic storytelling mirrors the middle phase of real-world couples therapy.

This book suits couples shaken by an emotional, workplace or online affair, especially when one partner insists it was "just a friendship". It speaks to the pain of feeling betrayed without clear proof of a physical affair, and to involved partners who never meant to cross a line. Its research-led style fits readers who want evidence alongside reassurance.

The core strength is Glass's clear, evidence-based account of how affairs begin. The main outcome is a shared understanding of where the walls and windows in your relationship went wrong, and a plan to put them right. If discovery is recent, start with Infidelity: A Survival Guide, read Glass to understand what happened, then turn to After the Affair to rebuild.