expression, noun
bookworm; bibliophile
A person who loves reading and spends a lot of time with books. Often used affectionately or self-deprecatingly.
彼女は本の虫で、一日中図書館にいる。
She's a bookworm and spends all day in the library.
子供の頃から本の虫だった。
I've been a bookworm since I was a child.
読書家 is a more formal term for a person who reads a lot, while 本の虫 is a colloquial, slightly humorous expression.
Literally 'book insect', a metaphorical expression likening a voracious reader to a bug that lives in books.