Softwood boards are named for their rough-sawn size before drying and planing. By the time a 1x10 reaches the shelf it measures 3/4 in thick and 9-1/4 in wide. Every plan on this site uses the actual size, because that is what your saw meets.
The common sizes
| Nominal | Actual |
|---|---|
| 1x2 | 3/4 x 1-1/2 |
| 1x4 | 3/4 x 3-1/2 |
| 1x6 | 3/4 x 5-1/2 |
| 1x8 | 3/4 x 7-1/4 |
| 1x10 | 3/4 x 9-1/4 |
| 1x12 | 3/4 x 11-1/4 |
| 2x4 | 1-1/2 x 3-1/2 |
| 2x6 | 1-1/2 x 5-1/2 |
| 4x4 | 3-1/2 x 3-1/2 |
Up to 6 in nominal, the actual width is 1/2 in less; from 8 in up it is 3/4 in less. Thickness is 3/4 in for "one-by" and 1-1/2 in for "two-by". The lumber size database has the full list including deck boards and hardwood quarter thicknesses.
What it means for a bookcase
A bookcase 30 in wide made from 1x10 has sides 3/4 in thick, so the shelves between them are 28-1/2 in long, not 30. Its depth is 9-1/4 in unless you rip a board narrower. The designer works all of this out from the actual sizes and refuses a shelf that would not fit between the sides.
Hardwood is different
Hardwood is sold rough by the quarter inch of thickness: 4/4 is a full inch rough, and about 13/16 in once surfaced on two sides. Widths and lengths are random and you pay by the board foot (a 1 in by 12 in by 12 in volume). The board feet calculator converts a board's size and price.
Check the board you bought
Boards vary by a sixteenth or so from one mill to the next, and wide boards can cup. Measure your actual stock before cutting joinery to fit it, especially dadoes, which need to match the shelf thickness exactly.