The resources which we found at Stoic Practice to be most helpful on Stoic meditation and spiritual exercises were Pierre Hadot's The Inner Citadel: The Meditations of Marcus Aurelius

and Bruce MacLennan's Notes on Marcus Aurelius which is derived from Hadot's book.

and the previously mentioned Stoic Spiritual Exercises by Elen Buzare which also seems to be based mostly on Hadot.