I bought a book whose title is very long.

先行詞が物の場合の関係代名詞所有格は whose もしくは of which

今日の例文は、I bought a book, the title of which is very long. でも可。
でも、of which は文語的で堅い表現。会話の時は、所有格そのものを使わずに、
I bought a book with a very long  title.や、I bought a long-titled book.の方が自然。

街角で見かけた英語（メルボルン）

booth「売店・仮設小屋」。観光客用のツアーバスチケット売り場です。