Book of Knights is a streamlined set of rules for Pendragon. It covers characters, Combat, and experience, and gives basic ideas for running the game, and the rules for personality traits. It is not, really, fully ready to go.
It is, however, an excellent player's book. It includes knightly character generation from most places in the isles. It doesn't cover non-knights to any depth, though.
It also is an upgrade to the combat rules in Pendragon 4th ed; the additional options add quite a lot.
It also has upgraded the character generation system, slightly.
Essentially, it is Pendragon 4.1 Light... and I reccomend it in addition to Pendragon 4th edition, or as a first taste for the timid.