This is the 86th most frequent Hungarian word.
közül
"From among" or "out of."
Here, 'közül' is used to denote selection from a group, 'the books'.
A könyvek közül ezt választottam.
I chose this one from among the books.
In this sentence, 'közül' indicates a choice within a group, 'his friends'.
A barátai közül Péter volt a legkedvesebb.
Among his friends, Peter was the kindest.
'Közül' is used here to signify selection from a set, 'the tasks'.