Depending on whether it's a hardcover or paperback, new books can retail anywhere from $15 to $25 generally (pre-tax). I might say $25 is a good amount for a gift card because it will generally buy one expensive book or partially cover two less expensive ones.
I totally agree with what your family member said about gift cards - if I get money, I just hoard it, but if it's a gift card it's not "real" so I can spend it with better peace of mind. That's exactly how gift cards get you. 😉