If you're going to read just one book, this one has been highly recommended by my psychotherapist friends and loved by my psychotherapist ex-partner!
Another great one one, that's written for a broader public, but brings together attachment styles and non-monogamy.
Below, a small collection of research papers on the topic of consensual non-monogamy (which is broader but includes polyamory):
.Consensual Nonmonogamy: Psychological Well-Being and Relationship Quality Correlates (2014)
.Comparing relationship quality across different types of romantic partners in polyamorous and monogamous relationships (2018)
.A comparison of sexual health history and practices among monogamous and consensually nonmonogamous sexual partners (2015) https://www.ncbi.nlm.nih.gov/m/pubmed/26395880/
.Jealousy, Consent, and Compersion Within Monogamous and Consensually Non-Monogamous Romantic Relationships (2019)
https://link.springer.com/article/10.1007%2Fs10508-018-1286-4
.A Critical Examination of Popular Assumptions About the Benefits and Outcomes of Monogamous Relationships (2012)
.Examining relationship quality across three types of relationship agreements (2016)
.Prevalence of Experiences With Consensual Nonmonogamous Relationships: Findings From Two National Samples of Single Americans (2017)
https://www.ncbi.nlm.nih.gov/m/pubmed/27096488/
.The association of an open relationship orientation with health and happiness in a sample of older US adults (2014)
https://www.tandfonline.com/doi/abs/10.1080/14681994.2014.976997
Those have special implications for the practice of therapy:
. Counseling Individuals Who Practice Consensual Non-monogamy (2020)
. They must be sick: consensual nonmonogamy through the eyes of psychotherapists (2022)
. Harmful and Helpful Therapy Practices With Consensually Non-Monogamous Clients: Toward an Inclusive Framework (2018)
*I lived in Brazil and speak Portuguese, in case you want to talk to me personally about that, I'd be glad.
OP, tough position but I understand where you are coming from. If you are really keen on working on your relationship, I recommend finding and reading Martha Kauppi's work. She is a marriage and family therapist and has some good resources for mono-poly relationships as well as for those struggling with their partner transitioning to polyamory. Her book "Polyamory: A Clinical Toolkit for Therapists (and Their Clients)" has in-depth, helpful guides and worksheets. Really thorough and amazing stuff. See here: https://www.amazon.com/gp/product/B08YWW1JGD/ref=dbs_a_def_rwt_bibl_vppi_i0
Her recent featured podcasts have been enlightening. I really liked her conversation with Dr. Joe Kort, which you can find here: https://joekort.com/martha-kauppi-on-successful-polyamory/
As well as this one on the Couples Therapy Couch: https://couplestherapistcouch.libsyn.com/086-martha-kaupii-on-open-relationships
Good luck!