The best-looking painted oak bookcases are made with a tight-grained wood that does not have any large knots.  In general, you should paint a bookshelf or bookcase to enhance the natural beauty of the wood rather than to cover up an inferior quality wood. There are many  options when choosing the right wood;

Fir or clear pine

Clear Douglas fir is relatively inexpensive and abundant in supply.  Don’t exclude the use of clear pine (wood without knots) for the purpose of bookcase construction. White pine from the North and Mid-West of the USA has long been used in the construction of beautiful painted furniture and is particularly good when constructing a wooden bookcase.

Poplar

Poplar is a lightweight hardwood that grows in the mid-Atlantic and Southeastern regions of the United States. Also called yellow poplar or tulip tree, this wood has a tight grain. As with the two above-mentioned softwoods, yellow poplar has good tensile strength, a necessary characteristic for any bookcase.

Veneer Plywood

Plywood can vary in quality, but generally a good quality birch style veneer plywood will provide a long life, lasting value and take coats of paint well.To insure a strong shelf, veneer plywood should be 3/4-inch thick, and the shelf should not exceed 4 feet without a support.Before painting, any strips of veneer should be cut and added to the fresh edges.Of course, woods such as cherry, maple and oak re perfectly suitable as well.

Salvage Wood

A great option for your bookcase would be some reclaimed or salvage wood. Try sanding it with a belt sander or palm sander to improve the quality of the wood surface.

Painted Maple

If you can find it, there’s a cheaper grade of maple that is quite popular for use as a painted surface. Maple is a hard, durable, strong wood, also prized as a choice material for cabinets and floors.

Particle Board

For a budget minded option, choose 3/4 inch particle board as your base material. It is inexpensive and can be cut and used in the same way as plywood, although it is not as strong as plywood. Several coats of spray paint can produce a very shiny surface, which will hide the fact that the bookcase is made with particle board.In summary, the best-looking painted wood bookcases are made with a tight-grained wood without any significant knots or blemishes. In general, you should paint a bookshelf or bookcase to enhance the natural beauty of the wood rather than to cover up an inferior quality wood.