書斎に

in the study

→ 書斎
phrase Uncommon #18,803
in the study 書斎 means “study” or “home office,” and に marks the location where something exists or happens. With movement verbs, に can instead mean “to the study.”
JA: 父は書斎にいます。
EN: My father is in the study.

彼の書斎にがたくさんある

There are a lot of books in his study.

書斎に一つ

One in the study.

— アイム・スティル・ヒア (2024)

俺は先に書斎に行ってる

I'll go to the study ahead of you.

— 箪笥<たんす> (2003)

紅茶を書斎に頼む

Have some tea sent to the study.

— レベッカ (1940)

書斎に先に行って待ってくれ

Go to the study ahead of me and wait for me.

— ラスト、コーション (2007)

書類書斎にある

The documents are in the study.

— ラスト、コーション (2007)

一度会ったけど家に居ないが多い戻って書斎に居るので会う機会がないの

We’ve met once, but they’re often not home. Even when they are, they’re usually in the study, so I rarely get a chance to see them.

— ラスト、コーション (2007)