I decided to read this book because someone I was interested in had read it.
Someone I was interested in read this book so I wanted to read it too.
I decided to read this book because the girl I'm interested in has read it.
I decided to read this book because.
この本を読むことにしたのは、
someone I was interested in had read it.
気になる人が読んでいたからです。
Someone I was interested in read this book so
気になる人はこの本を読んでいたから
I wanted to read it too.
私も読みたかった。
"The boy I'm interested in"/"The girl I'm interested in"は自然な「気になる子」になります。
The reason why I started reading this book is because a girl I like was reading the same book.
この場合、次のような言い方ができますよ。
ーThe reason why I started reading this book is because a girl I like was reading the same book.
「この本を読み始めた理由は、気になる女の子が同じ本を読んでいたからです」
a girl I like で「気になる女の子」
ご参考まで！
アイルランド 
カナダ 
