A general rule of thumb is that the best material for making oak bookcases are tight-grained without 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 several  options when choosing the right wood;

Fir or clear pine

Our first selection (clear Douglas pine) is both plentiful and abundant. Also consider clear pine (that means no knots) for building a bookcase.  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. Tight grained, this excellent material is also called yellow popular or tulip tree. 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.Strips of veneer need to be cut and applied to the sawed edges before painting.Of course, woods such as cherry, maple and oak re perfectly suitable as well.

Salvage Wood

Used wood can be converted into a bookcase and then painted for a smooth finish. Try sanding it with a belt sander or palm sander to improve the quality of the wood surface.

Painted Maple

Some lumberyards sell a slightly cheaper grade of maple that has found popularity as a painted surface. Maple is a hard, durable, strong wood, also prized as a choice material for cabinets and floors.

Particle Board

To save money, make a bookcase out of 3/4-inch particle board.It is generally as versatile of plywood (but perhaps less strong), but much less expensive. Several coats of spray paint can produce a very shiny surface, which will hide the fact that the bookcase is made with particle board.To summarise, 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.