。
The girl reading a book is Kate.
Literal
Book [object-を] read-[ongoing-ている] girl [topic-は] Kate [is-だ].
A textbook relative-clause sentence. 本を読んでいる ('(who) is reading a book') is a relative clause modifying 少女 ('girl'), with no relative pronoun — Japanese simply places the modifying clause directly before the noun. Then 少女はケートだ ('the girl is Kate'). 少女 is a somewhat literary word for 'young girl'; ケート is the katakana rendering of the English name Kate.