There are many types of paper available for the interior of your book. They differ in texture, color, opacity, thickness (PPI), etc.
Paper may be smooth or rough and glossy or dull. Smooth, dull paper is common for books containing mostly black text. Smooth, glossy paper is better for books containing bright colors and/or many photographs.
Most books use white or natural (a.k.a. beige, cream, off-white) paper, with natural usually being more expensive. Natural is the best option for books containing mostly text and intended to be read for long periods (e.g., novels, nonfiction) because the natural color is less harsh on the reader’s eyes. White is best for poetry books, cookbooks, and other books that are likely to be read for short intervals, as well as for books containing more photographs than text.
A paper’s thickness and opacity are dependent on its basis weight. Thickness and opacity increase with basis weight, as does cost. 50# to 60# papers are common in the offset book-printing industry (these offset basis weights are equivalent to 20# and 24# office (a.k.a. bond) papers, respectively).