Free ≠ Frei
One can usually not use the word frei (which mostly means “free” in the sense of independent, unbound, without handicap or unoccupied).
Rather, one would either use the word kostenlos (literally “costless”) or umsonst. Note that the word umsonst also has the meaning of “for nothing” (in the sense of a wasted effort) and thus it’s safer to use for free products rather than services.
Die Bücher sind kostenlos. – The books are free.
Das sind kostenlose Bücher. – Those are free books.
Die ganze Mühe war umsonst. – The whole effort was for nothing.
Other Common Ways to Say Free
Another word used to indicate that something does not cost money is gratis. You may see this word when shopping to indicate a free gift, or that something is complementary.