The only book written exclusively on MoE is: Heathcote & Bolton, "Drama for Learning"

For good introductions to Process drama: Bowell & Heap, "Planning Process Drama" and O'Neil & Lambert, "Drama Structures"

Morgan & Saxton, "Teaching Drama" is also a very useful introduction to drama teaching.

Bolton, "The Dorothy Heathcote Story" has a good chapter on MoE and is a brilliant read

Wagner, "Drama as a Learning Medium" was written before DH invented MoE, but is a great introduction to her work.