thought the world of that girl

東野圭吾の "The Devotion of Suspect X" を読んでいます。
'Does a Mr Kuniaki Kudo come here often?' he asked next, changing his line of questioning.
'Mister Kudo? From the printing company?'
'That's right.'
'Quite often, yes. Though not so much recently.' Sugimura tilted her head.
'Has Mr Kudo done something?'
'No, no. We've just heard that he was one of Yasuko Hanaoka's regulars back when she was a hostess.'
Sugimura's lips softened and she nodded. 'He was. He thought the world of that girl.'
'Were they seeing each other outside the nightclub?'
刑事が容疑者が元ホステスとして働いていた時の馴染み客について探っているところです。
文脈から "thought the world of that girl" の意味は凡そ見当は付きますが、"though the world of" は慣用句の様なので辞書を見ます。

・Oxford English Dictionary: Have a very high regard for (someone): He thought the world of his sisters and brother.

・Collins Dictionary: to be extremely fond of or hold in very high esteem: Very quiet woman but a good worker, Mary and John think the world of her.

・McGraw-Hill Dictionary of American Idioms and Phrasal Verbs: to be very fond of someone or something: The old lady thinks the world of her cats.

caper

東野圭吾の "The Devotion of Suspect X" を読んでいます。
"We did spend a lot of time and manpower questioning people around the station here. If your theory's right, that was all wasted time."
"Not a waste, per se. After all, the bicycle was stolen from her. But I don't think this case is simple enough that knowing that fact will do you any good at all. No, our littel caper was constructed far more craftily and with greater precision than that."
"caper" と言うと西洋料理に添え物（特にスモークサーモンには必ず付く）として出て来る見かけは小さな豆のような、食感は独特で豆ではないと直ぐに分かりますが、食べ物を思い浮かべますが、上記の "caper" 当然別物です。辞書を見ます。
・Oxford English Dictionary: An activity or escapade, typically one that is illicit or ridiculous.: Now he may face the full 10 years, plus punishment for the grave-robbing caper.

・Collins Dictionary: A dishonest or illegal activity can be referred to as a caper.; She served six months in prison for the helicopter caper.

・Cambridge English Dictionary: an illegal, unusual, or entertaining activity: The whole incident started as an innocent caper.
なるほどここでは死体発見場所の近くに残されていた自転車盗難事件のことですね。

prepping

東野圭吾の 'The Devotion of Suspect X' を読んでいます。私が住んでいる地区が舞台になっているので実際にそんな事件があったのかと思う気になります。
When they reached the Edogawa police station, Mamiya was standing by the heater, warming his hands. Mamiya was their division chief in criminal affairs. Several men -- probably local homicide -- were scurrying around him, prepping the room to serve as investigation headquarters.
上の引用個所に出てきた "prepping" は文脈からして "preparing" と同じ意味の様ですが、辞書で確認します。

・Oxford English Dictionary: Prepare (something); make ready.: scores of volunteers help prep the food
Origin 1920s: abbreviation of prepare or preparation.

・Collins Dictionary: If you prep something, you prepare it.: After prepping the boat, they sailed it down to Carloforte.

・Cambridge English Dictionary: to prepare yourself or someone else to do something: She said she was prepping for her new movie.
やはり語源は "prepare" でしたね。

John Doe

東野圭吾の 'The Devotion of Suspect X' を読み始めました。森下だの篠崎の地名が出て来たので、宮部みゆきと同様に江東区か江戸川区で育った作家かと思いましたが、大阪生まれで東京に移ったのは作家になってからだとWikipediaに書かれていました。最初に引用するのは殺人事件後に死体が発見された後の個所からです。
'There was a bicycle nearby. Some people from the local station in Edogawa already came to pick it up.'
'A bicycle? Probably junk somebody threw out.'
'It was a little new for that. And both of the tires were flat. Someone put a hole in them with a nail or something.'
'Humph. The victim's, then?'
'Hard to say. It had a registration number on it, so we might be able to find out who owned it.'
'Well, I hope it was the victim's,' Kusanagi said. 'Or else this is going to be a real tricky one. Heaven or hell.'
'How's that?'
'This your first John Doe, Kishi?'
Kishi nodded.
"John Doe" が身元不明の人を指したり、あるいは名無しの権兵衛とか、実名の代わりに使うのは知っていましたが、実際の文章で見たのは初めてだと思います。一応辞書の説明を見ます。

・Oxford English Dictionary: An anonymous party, typically the plaintiff, in a legal action.: For prosecutors, indicting John Does through their DNA might seem more than fair.

・Collins Dictionary: John Doe is used to refer to a man whose real name is not known or cannot be revealed, for example, for legal reasons.: In fact he's not John Doe anymore, we have an ID.