The Book of Eight Lands is an artifact that keeps an up-to-date record of all members of the ruling noble families. It also names which of those persons is currently the most worthy to rule. Traditionally, that person was always considered the heir to the throne.
The chaotic line of succession where the second-in-line might be your own brother but the heir is from an entirely different noble line (and worst, possibly a dwarf, so you know her rule is going to outlast your brother) eventually led to less scrupulous members of the ruling lines treating it as more of a hit list.
Steppengard reformed that system. He claimed some of the land belonging to some of the noble families (so the monarch has some lands of their own, since they're now distinct from the eight families) and established that Dassen is now a hereditary monarchy like some other nations. The throne will pass through his own heirs as the Lordship of each of the Eight Lands does through its own family, regardless of whom the Book names as most worthy. He and his wife then set to the arduous task of creating as many heirs as possible to ensure that his line won't be broken any time soon.
While Steppengard did ensure that the monarchy will stick with his own line, he also limited its power. Dassen's national affairs are now decided by council. Each of the Eight Lords has a vote, and the monarch's vote counts extra. Since the Eight Lords largely lead their own lands, each is typically represented at the council by a proxy rather than in person.
The Book of Eight Lands is available for viewing by any Dasseni citizen--one at a time, and under extremely strict guard, of course. It is well protected, but not actually locked away.
In addition to keeping records of nobles and naming the most worthy heir (now merely a curiosity), the Book has protective abilities. It is said to defend members of the ruling houses from some kinds of mind-affecting magic, as long as the Book itself remains in Dassen and the noble stands on Dasseni soil.
It is from this tradition that the superstition of putting dirt in ones shoes originates. Hey, you might be Iz's long lost great-niece twice removed, right? And having dirt from Dassen in your shoes is technically standing on its soil, right?
