It's the ultraviolet range (UV) of the sunlight that does damage to stuff like leather furniture and book spines. Modern glass can be bought that blocks most of the UV or, if you've already got new windows in, there are UV-blocking films that can be attached to existing glass. Also, if yo have any fluorescent lights be careful. They emit huge amounts of UV and you can buy cylinders that fit over the lamps that block the UV.

craigd's warning was spot-on.