Enthralling?

Hovering over book items in my inventory now produces a note that they are &quotEnthralling&quot, coupled with a number (either 3 or 4, from what I can see). What does this mean? Is it an update I missed?

There’s some discussion here (and another linked thread further upstream)

http://community.failbettergames.com/topic21627-enthralling--a-new-quality.aspx