This is the 3414th most frequent Polish word.
wziął
"Took."
'Wziął' here is the past tense form of the verb 'wziąć', meaning 'to take', referring to an action where Janek took possession of the book.
Janek wziął książkę ze stołu.
Janek took the book from the table.
'Wziął' in this sentence acts as part of the phrase 'wziąć się za coś', which means 'to start doing something' energetically or decisively.
Wziął się za naukę przed egzaminem.
He got down to studying before the exam.
Here 'wziął' illustrates taking responsibility, emphasizing assuming or accepting ownership of actions or decisions.