Sakhya (John Cort, Fall 2004)
1. There is a wonderful book called “Preyo Bhakti Rasaarnava,” which basically consists of Bengali poetry with numerous Sanskrit quotations, written by one Nayananda Thakur in 1731 CE. It is a detailed theological account of Krishna’s relation to the cowherds of Vraja, ala Gaudiya Vaisnavism.
2. There’s a (very) brief discussion of sakhya (with equally brief examples from Ramprasad Sen’s poetry) in Julius Lipner’s “Hindus: Their Beliefs and Practices”. It’s part of a longer discussion of bhakti types, models and themes (pp. 307-324).
3. See David Haberman’s translation of _The Bhaktirasamrtasindhu of Rupa Gosvamin_ (New Delhi: Indira Gandhi National Centre for the Arts; Delhi: Motilal Banarsidass, 2003). Rupa speaks in great detail in this famous work about sakhya and srngara rasas, as well as the other rasas.
4. Chapters 2, 8 and 14 of Ujjvala-nilamani by Rupa Goswami contain relevant material to your interest.
Chapter 2 is the section on “nayaka-sahaya”. Chapter 8 is called “sakhi-prakaranam.” Chapter 14 is about “sthayi-bhavas”, but in its definition of “pranaya” adds the element of friendship to other elements of passionate love as a step in the deepening of love.
This friendship is of four kinds–maitram, sakhyam, sumaitram and susakhyam.
Chapters 2 and 8 (with three commentaries), as well as Bhakti-rasamrita-sindhu (mula-matram) are available in transliterated Sanskrit form at http://www.granthamandira.org.