I gave my mom's book to a friend for his birthday
I am surprised to hear that. Was your mom okay with that?
I didn't tell her, but he will read the book, I'm sure he's got time for it.
It's nice that you thought of your friend on his birthday, but it isn't right that you have him your mom's book. Your mom will feel very upset when she finds out that you gave her book away without asking.
WTF! He can read the book and give it back afterwards and it's not hurtful to ask. How can a book be valuable anyway? Reading is important though.
Usually people do not give things that belong to others away to other people. You have no right to do that really.