Why are the Perfection of Wisdom sutras seen as a coherent collection?
All of these sutras are on the subject of wisdom, or "voidness", "sunyata", or the "emptiness".


Śūnyatā
Introduction to Meditation on Voidness (Emptiness).


There are many good teachings on the concept; here's one: Advice for Studying Voidness (Emptiness)

Every school has its own teachings on the subject. Indeed, all of them (I think) advocate meditation on the concept at some point. One form of Mahamudara meditation advocates using it from the beginning: Mahamudra Meditation Manual.

Good stuff!