Why are the Perfection of Wisdom sutras seen as a coherent collection? The Diamond Sutra, Heart Sutra, the Perfection of Wisdom Sutra in 8000 lines and others are grouped together as Prajnaparamita (perfection of wisdom). Why is this? 

Why are they seen as a coherent collection? Are they thematically similar or is the link because they were written at a particular geographic and historical period? Are they maybe linked because of they a reaction against other schools of Buddhist thought at that time? Should they be linked together at all?
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!