Bible paper
Also called India paper, Oxford India paper, thin paper, thin opaque offset
A very thin, light paper, generally between 30 and 50 gsm, yet opaque and strong, made with no mechanical pulp. It is used for works with a great many pages: bibles and missals, dictionaries, law codes, collected works such as the Pléiade editions. It is not onionskin, which is thin too but translucent.
In practice
Its weak point is show-through, and it is handled when the book is typeset. Opaque as it is for its weight, it lets the reverse show: choose a typeface of moderate weight, avoid solids and very black display headings, and set the lines of the recto to fall on those of the verso, what typesetters call backing up in register. Ask for a proof printed on both sides, on the paper itself.
It keeps a thousand-page book easy to handle: the spine stays slim and the volume light, without splitting the text into several volumes. The spine is worked out from the exact caliper of this paper, which the printer will give you.
It suits photographs poorly: with no coating, it absorbs the ink, and pictures lose contrast and sharpness. For an illustrated work, gather the pictures in a plate section on another paper.
It needs a printer who can run it, since a sheet this light creases and handles badly, and few keep it in stock. Plan for a sewn binding: glue alone holds poorly on paper this thin, and the book must open without cracking.