The text block is the body of the book, excluding all the papers added by the bookbinder, such as the endpapers. Traditionally, a book is made up of sheets of paper (or, once upon a time, parchment), folded into sections (or gatherings). Each page in a section is called a leaf, with a recto (the “front”, or the side that is on your right when a book is opened, at least in western bindings) and a verso (the back, or the side that is on your left).
The sections are stitched together with thread to form the text block. The thread was once often stitched around cotton tapes or hemp cords (thicker than string, thinner than rope), which helped to strengthen the structure. Cords will still visible on the spines of older books, as raised bands—the leather or fabric covering was moulded around the cords to create a decorative element. But beware—sometimes those raised bands are false! Cheaper bindings often imitated the appearance of raised bands to make the book look fancier than it really was.
Once stitched, the text block was often rounded, to achieve that nice convex shape. This involves bashing the spine with a hammer to make a kind of mushroom shape. This part of the bookbinding process is really excellent for those with anger-management issues. You can’t go crazy though, because the trick is to get it the right shape—not too flat, not too steep, with just the right amount of shoulder (the part that overhangs the main text block, like your own shoulders overhang your torso) into which to fit the covers.
The spine of the text block was often reinforced with a spine lining, often a loose-weave fabric called mull, or with layers of paper. The edges of the spine lining help to attach the book to its case, if the covers themselves are not stitched onto the text block.
Another word you might see used in relation to the text block is swell. This describes the increased thickness of the text block near the spine, due to the added bulk of the stitching. There are a lot of factors that affect the degree of swell, which we won’t go into here, but suffice to say it is one of the things the beginner often stuffs up entirely.
The edges of the text block have usually been trimmed to give an even edge. Sometimes, however, you may see books with a deckle edge. In traditional papermaking, the deckle is the frame used to give the paper its shape during the moulding process. A bit of paper pulp seeps in under the deckle, giving that uneven edge. Sometimes bookbinders keep this edge for its decorative effect. (NB Modern papers are not made with a mould and deckle, but you may come across papers made to imitate this effect—again, to make the book look fancier than it really is).