So I tried to write a good summary of this series half a dozen times and could never quite get the right wording and vagueness that would compel an interested party to read further, but yet not spoil any of the books. Lacking the words I needed I went to Seanan's website for guidance, or in this case a nice neat summary.
1. Any creature whose existence has been suggested but not proven scientifically. Term officially coined by cryptozoologist John E. Wall in 1983.
2. That thing that's getting ready to eat your head.
3. See also: "monster."
The Covenant of St. George was founded to uphold one simple ideal: anything that was not present on the Ark—anything they deemed "unnatural"—needed to be destroyed. Monsters. Creatures of myth and legend. All of them would be wiped from the Earth in the name of Man's dominion. Unfortunately for them, not all the monsters agreed with this plan...and neither did all the human beings.
After their rather abrupt departure from the Covenant, Alexander and Enid Healy found themselves alone in the world, but with a simple mission of their own: to protect the cryptids of the world from those who would harm them without just cause. It was a cause that would eventually claim both their lives, leaving their children, and their childrens' children, to take up the fight. Now in the modern day, their descendants struggle to stay beneath the Covenant's radar, while defending the cryptids from humanity—and humanity from the cryptids."
Unlike most series this one is divided up between two different, but related main characters, siblings Verity and Alex. For books 1, 2 and 5 you will be with the ballroom dance loving Verity, and for books 3 and 4 with the reptile loving Alex. Since I am more into reptiles then ballroom dancing, I found Alex's books far more interesting. It could also be his actual love for being a cryptozoologist, unlike his sister who treated it like a day job. No matter which sibling the reader is with, there is plenty of danger, excitement, and weirdness.
My final recommendation: This series is well worth the read, if nothing else for the cryptid world that Seanan weaves, that and the talking mice. Yes, talking mice, and they are awesome.