References – SN 22.5: Concentration 
Readings by Ajahn Karuṇadhammo: [Aggregates]
Satipaṭṭhāna: The Direct Path to Realization by Venerable Analayo, pp. 202-207.
Sutta: SN 22.5, Khandhasaṁyutta, “Concentration.”
Sutta: SN 22.29, Khandhasaṁyutta, “Delight.”
Sutta: SN 5.1, Bhikkhunīsaṁyutta, “Āḷavikā.”
Sutta: SN 5.9, Bhikkhunīsaṁyutta, “Selā.”