Open any European sales advert and you will meet a wall of abbreviations: KWPN, Hanoverian, Oldenburg, Danish Warmblood, PRE. These are the studbooks, and understanding what they are saves a lot of confusion.
Here is the key idea that trips up most newcomers. The European warmblood “breeds” are not closed breeds in the way a Thoroughbred or an Arabian is. They are open studbooks, which means they are really selection systems: breeding organisations that set standards, inspect and grade horses, and admit good genetics from wherever they come, including from each other. The same successful stallion can have registered offspring in several studbooks at once. A Dutch KWPN horse and a German Hanoverian can be more closely related than two horses within either one. So when an advert tells you the studbook, it is telling you which quality-control system produced the horse as much as anything about its genes.
The warmblood studbooks dominate modern competition dressage. The KWPN of the Netherlands, the German books of Hanover, Oldenburg and Westphalia, and the Danish Warmblood lead the world rankings between them, each with its own character and reputation but all breeding toward the same modern sport horse. They share so much blood that a buyer is usually better off judging the individual horse than agonising over which book it belongs to.
The Iberian horses are the real exception. The Spanish PRE, often called the Andalusian, and the Portuguese Lusitano come from closed studbooks and centuries of selection for collection: the ability to sit and carry weight behind, which is exactly the hard part of dressage. They tend to be brave, people-loving, and easy to get along with, which makes them popular with amateurs. In return they often give up ground in the big extended trots that the modern tests reward, though sport-focused Iberian breeders have narrowed that gap considerably.
Do not shop for a studbook. Shop for a horse, and let the studbook tell you where to look and what to check. The best horse for you has come out of every book named here.