According to my book (Mastering Arabic 1, Jane Wightwick and Mahmoud Gaafar), the possessive ending is with u: kitaabuka (your book, male), kitaabuki (your book, female), kitaabuhu (his book), kitaabuhaa (her book). (if nominative declension)
We should learn declension to determine the correct vowel. If genitive, I think it should be fii kitaabihaa (in her book). If accusative, "I see her book", kitaabahaa.