Last week, all the pages of my book were stained with vegetable oil. I have tried some methods, but they don't seem to work. Please do suggest any effective methods of removing such stains.
I think that you're out of luck. The only way to dissolve the oil will also dissolve any ink on the pages of the book and probably will destroy the binding if it is case-bound (hard cover) as well.
If the contents of the book are valuable, you can store it in the refrigerator to prevent the oil from becoming rancid until you transcribe the content.
If you can, replace the book. It is unrecoverable. (No pun intended.)
I would try 99% isopropyl alcohol, or denatured ethanol if you have gloves and good ventilation. First, test to see whether it removes the ink. If not, this is a great solvent for oil. When I get oil on cotton, I spray some IPA onto it and blot it off with a tissue. The same technique should work with the book, unless the ink can't handle it.
I would do a few pages at a time for the first pass--put a couple paper towels under the pages, spray the pages with IPA and let sit for a few minutes so the oil and water mixes. Then blot to remove the oil/water mix. You'll need to repeat this several times.