Books

Are you referring to the way the pages cluster together in sections?

That’s a consequence of the way the book is bound: the sheets are printed on large 4x4 press sheets that are then folded in half four times, stitched through the folds to the fabric spine in "signatures" of 32 printed pages, and trimmed on the outside edges. One consequence is that the finished pages in each signature are each a slightly different width if you take into account the part curving into the fold; and if the pages are bent into a different orientation, this difference in widths becomes apparent.

This is the more traditional method of bookbinding, used on your edition of LOTR (I’ve got the same edition.) More typical mass-market paperbacks are bound using "perfect binding", in which the cut edges of the pages are glued directly to the spine instead of being folded into signatures and stitched. The traditional method is more durable and flexible, without the issue of spine cracking that glued, perfect-bound books are prone to.

The difference is a bit more evident in the soft-cover binding used for this edition (as opposed to the hard covers more commonly used for traditionally-bound books), because the soft cover allows you to bend the pages of the whole book block (emphasizing the signatures) more than a hard cover would permit.