They tell the Coming-of-Age Story of Eff Rothmer, a supposedly unlucky thirteenth child, and her twin brother, Lan, a seventh son of a seventh son. They and their family live out on the edge of the relatively safe part of the United States of Columbia, watching as every year more and more people brave the dangerous wildlife to settle the frontier.
This series provides examples of:
- Aerith and Bob: The names of the baby Medusa Lizards in the 3rd book are Sthenno, Euryale and Fred.
- Alternate History: It's fairly clear that while we are on Earth, history has not progressed in quite the same way. For instance, there are no Native Americans.
- Awesome, but Impractical: Cathayan magic works by pooling the magical energies of dozens of mages and channeling them through a single adept. This allows for phenomenal feats of magic, like leveling mountains or damming rivers, but it's kind of useless when you just want to light the cookfire.
- Badass Normal: Brant, as he doesn't even use magic, but still managed to save a dozen people from certain death.
- Boring, but Practical: Avrupan magic may not be quite as impressive as Aphrikan or Hijero-Cathayan, but it is the king of useful little spells like making heavy things lighter, speeding up travel, or lighting a fire.
- Different States of America: The United States in this book is one where its westward expansion was halted by the presence of dragons and other magical creatures.
- Exotic Weapon Supremacy: Or, rather, Exotic Magic Supremacy. Avrupan magic is never as effective as Aphrikan or Hijero-Cathayan.
- Massive Numbered Siblings: Since seventh sons are powerful, many families have numerous children in pursuit of one.
- Non-Malicious Monster: None of the monsters from the West are interested in anything other than survival.
- Numerological Motif: Seventh sons and thirteenth children.
- Only Known by Initials: Eff is actually short for Francine.
- Prehistoric Monster: Woolly mammoths and rhinoceroses appear in the story.
- River of Insanity: The Far West functions as this, as very few people who venture out beyond the Great Plains ever come back, and none manage to keep their sanity.
- 13 Is Unlucky: Eff is the thirteenth child in the family, which is considered unlucky. That's why her extended family hates her.
- World of Action Girls: Eff, Miss Ochiba and Professor Torgeson are just a few. Eff even says that the farther west you go, the less people care about what you are than what you can do.
- Wizarding School: Avrupan magic is a standard school subject from the age of 10. At the university level, all the schools mentioned seem to be exclusively schools of magic.