How to reduce or clean up vegetable oil that spilled on books?

My partner and I have been moving house recently, and in all the action of packing, some cooking oil has spilled over a few of the books.

The books are irreplaceable, they are not made of any special materials, so standard paper and bindings so no fabric covers.

I have tried using washing up liquid in solution in small quantities to rub on the books, and left to dry for a day in the sun. This worked on the hard covers but still there is moist oil residue.

Is there anything else I can do to help recover some of the damage to the oil stains on the pages and soft covers?