There are countless options for the interior (or inside) of a book.
Colors
A standard black-and-white book prints 1/c (shorthand for “one color”). This can also be indicated as 1/1. This means “one color both sides,” which means printing on both sides of the page.
1/1 and 1/c are both correct, and preference depends on the person. (I tend to use both, depending on the book)
Full-color books (like picture books) print 4/c (four color) or 4/4
Books can also print with additional colors, like 2/c. This can be a black-and-white book with chapter headings/artwork in a different color.
Various specs
Case – Hard exterior of a hardcover book, made of boards. Paper or fabric is glued to it, making the boards unseen.
Diecut – Portion of a cover or jacket is cut out to reveal the page underneath
Endpaper – the first page when opening a hardcover book. These pages are glued to the inside of the front and back covers for support. Can be plain white, or printed in any color
French flaps – Flaps on a paperback cover that mimic a book jacket
Gilding – Metallic foil on the edges of the book (can be gold or silver)
Headband/tailbands – Small, decorative piece of cotton at the top/bottom at the spine
Insert – Colored pages inside a book. Usually on a glossier paper
Sides – the front and back covers of the book (does not include the spine, which is separate)
Slipcase – A decorative box for display. Most often used for box sets, or special edition books
Step-back cover – Cutting the edge of a paperback cover short to show part of the page underneath
Spine – the side that faces out when on a bookshelf
Staining – Applying a color to the edges of the book (like gilding, but uses ink rather than foil). Can be solid or patterned