Mount summon spells are no longer listed in the spell book under General.
To summon a mount, bring up the paperdoll (default "c" key) and select the pets tab at the bottom. There will be two tabs on the new screen, one for mounts and one for non-combat pets. Select the mounts tab and all of the mounts that you've acquired will be there. You can drag the buttons to your action bars to use in the same fashion as dragging the spell button from your spell book or the item from your inventory worked before.