For a better understanding of differentiation-based relationship therapy, see the work of Ellyn Bader and Peter Pearson at The Couples Institute, the contributions of David Scharch, founder of Crucible Therapy, and the pioneering ideas of Murray Bowen and his views on The Differentiation of Self.
My top two book recommendations on the topic are Passionate Marriage by David Schnarch, and Extraordinary Relationships by Roberta M. Gilbert.
You can also sample my book , The Re-Connection Handbook For Couples here, and my R3 Relationship Masterclass here.