How many books? Any chance of finding others with books they'd like to donate? Once you get past a small parcel, it's worth looking at shipping agents*, I donated some computer parts to a group sending to a schools project a few years ago, they were aiming for a minimum of 30kg, though ended up sending several times that. I did hear the cost, I can't remember what it was but it wasn't outrageous.
* Despite the name, most small consignments fly.
