Ending prepositions.

Hi all and thanks for reading and helping me;

I have heard a "rule" that says that you can not end a sentence by using prepositions; and I'm wondering if this one is always true.

If so, is this sentence wrong?:

The book you gave me is not the one I asked you for.

This one ends in -for; should I replace it for another grammatical structure not ending in preposition?

Thanks in advance!!!