kies

Kies does duty as both the question word and the relative pronoun, so one form covers "whose book is this" and "the man whose house". It never takes -j or -n, because it is already possessive: any plural or accusative ending goes on the noun it introduces, as in kies voĉon vi aŭdis. It sits in the possessive column of the correlative table alongside ties, ĉies, ies and nenies - learn the column once and you have five words rather than one.