"I was reading a book about your grandfather."
Translation:Četla jsem knihu o tvém dědečkovi.
That sounds really strange. It isn't really used. Better would be if there was a name following "dědečku", but just "o tvém děděčku" is weird. Point 4 here https://prirucka.ujc.cas.cz/?slovo=d%C4%9Bde%C4%8Dek#bref3 agrees that it normally is used when a name follows.