There's been discussion on Dragonsfoot about whether magic-users who fall into rivers or swamps risk having their spellbooks destroyed. My own experience with cheap, modern paperbacks is that they can get wet without being instantly destroyed, but if not properly dried out, they risk getting moldy. In fact, my own experience is that humidity, without actual immersion, runs a risk of destroying books with mold. This is why I no longer have a good copy of the PHB.
Would I roll a saving throw for a spellbook that's been dunked in a swamp? Probably not, although I might give a 1 in 6 chance of being infested with mold if the MU didn't take appropriate steps afterward. But it does raise the issue of item saves. I hate tons of die rolling, so I think I'd just use a single opposed d6 roll and adjust the player's result for individual items based on their intrinsic qualities. If I felt like it, I might let a player roll dice equal to their character's HD; they can assign each die result to one item, with the last die being a catch-all for all remaining items, comparing each result to my d6 roll. That way, some items will make their save and some will fail, but it's fairly quick to adjudicate.