"Souls United" by Ann Merivale is in my opinion the most complete and comprehensive work on the subject of what has been called various terms such as "twin souls," "twin flames," "divine complements" (Marie Corelli) or "The birth-bond" (Dante Gabriel Rossetti). It has been something that I've studied my whole life because I believe that my parents were of this type of relationship. Initially I did not have a language to describe my experience. As a young adult, my first introduction to the subject was through the Victorian writer Marie Corelli, in her books "Romance of Two Worlds," "Ardath," and "The Life Everlasting." Gradually, over time, I began to do research and read every book I could find on the subject. What I've discovered over the years is that there have been a few excellent books written and many that were rather limited in their scope.
Ann Merivale does a thorough and professional job of covering previous works on the subject as well as covering historical background for the concept of the twin soul, going all the way back to the writings of Plato and other cultures to develop a foundation for this topic. Ann Merivale also writes of historical people who likely were twin souls, such as Elizabeth Barrett Browning and Robert Browning, St. Francis and St. Clare, Pierre and Marie Curie, and more modern twin souls such as Peter and Eileen Caddy, Sri Aurobindo and The Mother, and Edgar Cayce and his family members.
The outstanding aspects of her book are:
1) Numerous case studies from her own practice and people she's personally interviewed to serve as experiential examples of this type of connection. All of the examples are from real life interviews and cases, and in my opinion are representative and realistic of how twin souls may manifest in this life.
2) A multi-dimensional view of how the twin soul relationship manifests in this life. This has been my own observation, so I found it affirming that Ann Merivale presented some alternate relationships to the more traditional and one-dimensional view that the twin soul relationship is the ultimate "romantic partnership." In her excellent book "Soul Mates and Twin Flames," Elizabeth Clare Prophet says,"...Now I have to tell you that sometimes and in some circumstances, because of such a difference between twin flames, God actually works it so that they embody as brother and sister, or as father and child, or as members of the same sex..." Ann Merivale's research agrees with this and in her case studies there are examples of twin souls who are best friends, twin sisters, same-sex twin souls (who are not always romantic partners), as well as a few examples of homosexual and heterosexual non-romantic but very bonded partnerships between twin souls.
3) Ann Merivale's research also has many examples of more traditional and romantic couple relationships drawn from her case studies and from modern public figures. Through these examples, the author presents some helpful applications for loving relationships for all couples and for those who believe themselves to be twin souls.
4) The final ways in which Ann Merivale's book excels is all the numerous references to other books on both twin soulship and spiritual subjects, so that her book is itself a reference work and resource. Thanks to "Souls United," I've had the great pleasure to be introduced to all of Joan Grant's work, which includes some beautiful descriptions on the twin soul relationship. I've also been introduced to authors like Anne and Daniel Meurois-Givaudan, Simon Bushnell, and the writings of Peter and Eileen Caddy, who founded Findhorn.