What function does the book spine serve?
The most crucial function of the spine is to provide a place to bind the pages together. No spine and a "book" is simply a block of paper. Its secondary function is to provide a place for the title and author to be displayed, so the book can be easily identified.
It should be noted that back is often used as a synonym for spine. Confusion can arise when people refer to the commonly used terms front or back covers, however the preferred terms for these covers are the upper cover and lower cover.
Depending on the type of binding used, a spine can be flat or ridged.
"That part of a book which is visible as it stands closed on the shelf; not uncommonly called in antiquarian parlance the backstrip, and sometimes called the back." -- ABC for Book Collectors
cocked -- when the spine is crooked or bent out of shape (often from poor storage).
The most-succinct definition of how to identify a book's spine probably comes from Tom Congalton and Dan Gregory of Between the Covers Rare Books, Inc.:
"The part of the book that faces you when it is properly shelved on a bookshelf."
Printing the title on the spine
Today, it is common to see a book's title printed on the spine -- in fact, commercial publishers consider that their greatest challenge is often to compress the information conveyed on a book's cover onto its spine, as most books only enjoy a few weeks spent "face-out" (positioned so that the front cover is visible) on a table or displayed on a shelf before spending the rest of its commercially viable life "spine out" on bookshelves.
But, it was not always so. Early shelving conventions were to place the book horizontally on the shelf, with the spine inward or perpendicalar to the shelf edge. Henry Petroski, in his excellent book on the development of the book through the ages, The Book on the Bookshelf, details how the half-title page has its origin as a label that could be removed from the book block and folded to display the title in front of the fore edge (the label being held in place between the front cover and the free endpaper (the endpaper that is not pasted onto the cover board.
The first illustration of a printing press and bookseller's shop appeared in a 1499 edition of Danse Macabre, published in Lyon by Mathias Huss. Interestingly, it shows no books shelved spine-out or vertically.
As book binding techniques improved, allowing books to be shelved vertically without causing damage, the convention for the title to be printed on the spine became established. However, the orientation of the title was not fixed for many centuries.
"Not surprisingly for a new practice, whether the title should be read up, down, or across the spine was not agreed upon. Indeed, lack of agreement in the matter persisted among English-speaking countries as late as the middle of the twentieth century, when books bound in Britain still tended to have their titles read up the spine, whereas those from America read down, conventions as opposite as driving on different sides of the road." -- Henry Petroski, The Book on the Bookshelf
Ultimately, the American convention of reading a title down the spine became the norm, but designers still like to vary the orientation for effect when the spine is wide enough.