I like the slave idea, but I can imagine the added irritation that would come from not only losing your spellbooks but actually having them running AWAY...
Let's see, something that would make sense for their natural peculiarities....
Acid etched, psionically sensitive crystals/gems which are then swallowed and held inside the Aboleth's gizzard? "Reading" them then involves hitting them with their natural psionic energy and absorbing the resultant auras (which just coincidentally takes as long as a wizard reading normal spellbooks of course).
Definitely some "ick" factor in getting your hands on them - which you can increase by saying the crystals needing to be external to the creature before giving hem energy (and thus reswallowed afterwards) and then having your PCs stumble across it while it's studying. Then the "aura" becomes some mystically colored and tainted water/air which the Aboleth then absorbs through its gills.
So how could a normal PC use these? A regular read magic wouldn't necessarily work because the point to gaining the spell is absorbing the aura and laying it down on a wizardly-trained mind (so other characters can't use them). Require the equivalent of 1 PP (either per use as a group, or per crystal/gem) each time they are used to create the aura/gas. Each crystal/gem is useable only once per day.
If you're feeling really RBDM, let them be able to gain the spell but let the gas be mildly poisonous or hallucinogenic with no save, amount of damage sustained equal to the number of spells (or spell levels) absorbed? In which case I might actually let other classes benefit from them if they are low level spells. Level 0 definitely - maybe spells up to level character level / 6 rounded up.