I too am having trouble finding Zernan.
If I refuse to give him the book, he steals it but there is no ambush, he just sits there. If I give him the book he leaves and I go to Candlekeep, meet Nandin, and he says to go find Zhurlong.
I go to Beregost, but Zhurlong has no relevant dialogue, and I can't find Zernan there.
Any suggestions?
BTW if you let Zernan steal the book, he sits there and you can kill him. He's carrying the note, but not the book, and if you do this there is no Nandin, and no Mysterious Man.
Thanks in advance
Vandervecken
EDIT: I should also add that you can present the wrong book at Candlekeep. There are two "I have the book you want" options. They look identical but the first fails and the second succeeds.
Can you post your WeiDU log please?