Naughty Bus. A lovely picture book about a toy bus. Also some bus companies do bus story books. We've got Ernie Excel Goes to School from Konectbus of Dereham, Norfolk and Plymouth City Bus do some too.
Not a book but we recently found a fab show called 'tayo the little bus'. There are a few episodes on YouTube, I think it's South Korean but we found English translations of the show. Our LO loves buses but there never seems to be many things on them, but he loves this show now. Just a shame its not widely shown.