When I bought a box of Pendragon stuff last year there was no 'Pendragon' core book, but there was a copy of The Book of Knights, which appears to be a version of Pendragon? I looked around online but I can not find anything useful on this book, not even a review. What is this book, and how does it differ from my Pendragon 5e book?

It's a short version of Pendragon (4th edition, I believe). It's a good start, but not Pendragon 5+

It's a short version of Pendragon (4th edition, I believe). It's a good start, but not Pendragon 5+

Got it. It looks handy to introduce new players.

Got it. It looks handy to introduce new players.

Except a lot of information in it is incorrect

Got it. It looks handy to introduce new players.
Except a lot of information in it is incorrect
VERY TRUE
Concept wise it is a good intro, but for specific rules you will NEED to double check to ensure that the information is still accurate.