Will a tower shield protect against dragon's breath,
magical rays, and the like? Can the user automatically
make a save against a fireball or even take no damage if
taking total cover behind the shield?
  It depends on the type of magical attack. The shield is a
piece of the user's equipment and attacking a creature’s
equipment is the same as attacking the creature.
  If the magical attack has the shield user as a specific target
(that is it does not merely affect the area that contains the shield
user but is aimed right at the shield user), the shield has no
effect. All rays fall into this category, as does any spell that has
a Target entry in its header and any spell that has an Effect
entry and affects creatures (provided, of course, that the
attacker can and does choose the shield user as a target).
  Magical attacks that fill areas (bursts, cones, cylinders, lines,
emanations, and spreads) are subject to all the rules for cover
on page 133 of the Player's Handbook. Note that spread effects
might be able to reach around the shield; if so, the shield
provides only one half cover.