Be fair: it's not the nourishment we're worried about, it's just the damage. After all...
by the RAW,
Swallow Whole
If a creature with this special attack begins its turn with an opponent held in its mouth (see Improved Grab), it can attempt a new grapple check (as though attempting to pin the opponent). If it succeeds, it swallows its prey, and the opponent takes bite damage. Unless otherwise noted, the opponent can be up to one size category smaller than the swallowing creature. Being swallowed has various consequences, depending on the creature doing the swallowing. A swallowed creature is considered to be grappled, while the creature that did the swallowing is not. A swallowed creature can try to cut its way free with any light slashing or piercing weapon (the amount of cutting damage required to get free is noted in the creature description), or it can just try to escape the grapple. The Armor Class of the interior of a creature that swallows whole is normally 10 + ½ its natural armor bonus, with no modifiers for size or Dexterity. If the swallowed creature escapes the grapple, success puts it back in the attacker’s mouth, where it may be bitten or swallowed again.
In other words, it doesn't [by default] do any damage at all!
Or, for that matter, absorb any nutrients whatsoever.
But, of course, most monsters do deal damage, and presumably eat-to-live, or suspension of disbelief would fail miserably.
Eating a creature immune to acid would do no good, except for the crushing to death. They'd be like, all fiber, and you'd get nothing useful from 'em.
Just don't eat them.
Constructs, too, are pretty inedible [assuming metal or stone here], except for things like dragons, which are stated as eating a diet including minerals.
Elementals? Well, no more than quenching your thirst or eating dirt. Which is what they are.
In short, here's the monsters I can find with a quick search that have swallow whole:
Dire Shark: Nothing but an animal; doesn't get nuffink.
Tendriculos: Your call-- eating an earth elemental might do it some good.
Dinosaur: Animal. Sorry.
Behir: Made of meat, but I think it deals electric damage on a swallow whole (?)
Nightshade: Undead evil! All calls are off! Eats everything!
Remorhaz: Made of meat = eat meat, not metal.
Tarrasque: It's the tarrasque. Your call.
Purple Worm: Big nasty purple thing, but still made of meat.
Gibbering Mouther: Pure chaos. Eats whatever you like.
That seems like way too small a list, so I must have missed half the MM; I just did a quick search for swallow whole on d20srd.
PS: By eats only animals, etc, I don't mean everything else is immune to damage. I'm referring to diets & delicacies here.