What kind of paint you choose to apply to your bookshelves is unimportant. Any kind will do the job. What is important is how you prime the bookshelves. The first thing that you should do is gently wipe the surface with mineral spirits. This will remove any oil that is on the surface and will allow future paint to adhere to the surface. Acrylic or oil, you must use a primer/sealer first. This seals the wood and provides adhesion. The finish coat will not soak into the wood, but will stick well to the primer. As a result, fewer coats are necessary. Acrylic paint is fine as a final coat, but oil paint results in a harder surface.