The dragons in the Chrestomanci series are native to Series Eight of the Related Worlds. Although they are a sapient species, they are at risk from poachers who slaughter them for their blood. Dragon's blood is a highly-prized, not to mention insanely dangerous - and for all these reasons, strictly controlled - magical ingredient; the addition of dragon's blood can boost the power of any spell. In its natural liquid form, it has a remarkably offensive odour and the tiniest amount is lethal on contact; it is usually stored as a dried powder, which is slightly less dangerous.
In The Lives of Christopher Chant, Christopher Chant gets to see an adult dragon up close. It is a magnificent sight, more than twice the size of a house, purplish-black with bright green eyes.