To get the Ancients Spellbook, open up your spellbook, and teleport to Edgeville using the home teleport button in the top left.
Once in Edgeville, walk west from the spot you teleported until you see the Lumbridge Sage just south of the General Store
When you see him, talk to him, and select the "Could you take me to the Desert Pyramid" option.
Once at the desert you will need to head south behind the pyramid.
There you will see the Ancient Altar.