Compound noun: Jigsaw puzzle.
Palapeli is a compound noun coming from pala which means ”bit” or ”piece” and peli which means ”game.” ”Piece game” is a matter-of-fact description of a puzzle, when you think about it.
In our example from the book shop sign, palapeli appears in the partitive plural, palapelejä, because it is modified by paljon, which means ”many.” Because ”many” is uncountable, the noun it modifies takes the partitive.
In our example sign, nuortenkirjoja means ”Young people’s books.” Kirjoja is partitive plural of kirja. Again, it it appears in partitive plural because it is an uncountable number on sale for 5 euros.