Any manuals on the development of "sympathetic joy" and or "gladness" apart from Visuddhimagga? As titled. I wonder if there are other methods of development outlined elsewhere?
There're quite a lot of essays on the Brahmavihara written by Vens. Thanissaro, Buddharakkhita, Siriwardhana, Nanamoli, etc. and of course in the Nikaya suttas themselves. See various links here and here