The first four Lost Books supposedly cover the time lost in between Black and Red. When I first read the first two Circle books, I didn't notice anything missing that was worth dragging back up again.
But Dekker is using the Lost Books as a marketing tool to bring teens into the Circle because the Lost Books are supposed to be teen fiction.
Chosenis not only a marketing tool, but a book used to introduce the Books of History\blood technique that is used conveniently later.
The Forest Guard is supposedly running low on recruits, so Thomas of Hunter is forced to allow teenagers into his army. But he wants to find the Lost Books of History before Teeleh finds them. Rumor has it that whoever has all seven rules all time.
So Thomas chooses four teenagers to the search for them. The way he chooses them is probably the highlight of the book. They play an interesting variation of football to find out who is strongest and wisest.
Johnis, Silvie, Darsal, and Billos are chosen to recover the Books of History. Johnis is immediately portrayed as the "chosen one", the special kid who can see things the other can't like Roush and Shataiki.
The book is very short, shorter than it looks. I think Dekker wanted to stretch the series out so the first letters of every book would spell CIRCLE. There isn't much substance to this book because of its length. The whole book is like a long introduction ending with a suspenseful showdown with Teeleh at the end.
The characters are already plain and stereotypical. There is already a romantic subplot introduced. There's already too much drama.
I know the Circle is Dekker's career work, but this one just doesn't wow me.