Assalamualaikum... I often see that the authors dedicate their books to their parents or beloved one.And one of the muftis in our country say that you can't dedicate anything to anyone as dedicate means sacrifice.And his logic is that every sacrifice should be for Allah.. So according to his it's a shrik.
Everything we do is for the sake of Allah (ﷻ) so the niyyah (intention) of an author writing a book should be that the writing of the book is for Allah's (ﷻ) sake and cause, not for a human's sake. I would agree with the Mufti in this sense that if the author wrote it for the sake of other than Allah (ﷻ) and then verbalized this in the 'dedication' page, it would be incorrect.
Qur'an 6:162: قل إن صلاتي ونسكي ومحياي ومماتي لله رب العالمين.
(Say, "Indeed, my prayer, my rites of sacrifice, my living and my dying are for Allah, Lord of the worlds).
That said, it is proven from the sunnah that you can do an act of charity for the sake of Allah (ﷻ) and then dedicate the reward of that to a specific person. (e.g., knowledge from a book that benefits people is an ongoing charity and per my understanding, it is fine to dedicate its reward to someone).
So in this sense, for an author to write a book for Allah's (ﷻ) sake and then use the 'dedication' page to say he wishes the reward for a loved one should be okay inshaAllah as I'm not aware of any pieces of evidence that indicate otherwise.
So it depends on how you are approaching the 'dedication' page. I hope this helps.
وعليكم السلام ورحمة الله وبركاته