。
She did nothing but read all day.
Literal
She [topic-は] all-day reading [only-ばかり] was-doing.
~ばかり ('nothing but / only') attaches to a noun (here 読書, 'reading') and combines with the verb する to give 'always doing only X.' The pattern ~ばかりしている picks out a single, near-exclusive activity — slightly disapproving in tone, suggesting that the speaker thinks the subject ought to be doing other things too. Adding 一日中 amplifies that: not just frequently doing nothing but reading, but for the entire day. 読書 is a Sino-Japanese noun ('reading' as an activity), used as a hobby term — 読書が趣味 ('reading is my hobby').