PBOs, also known as paperback originals, are simply books whose First Edition usually (though not always) is published as a paperback rather than a hardback. PBOs are quite common in genre fiction such as romance fiction and science fiction:
As the market for literary fiction shrinks, it is not at all uncommon to encounter even this type of fiction as a paperback original:
PBOs pose a number of problems for the average collector. Of particular concern are PBOs issued as mass market paperbacks (the type most frequently published for first-time genre authors, which one finds in airports, supermarkets, drugstores and like venues). These usually are printed on cheap paper that discolors and disintegrates fairly rapidly over time.
The spine of mass market paperbacks also is a major concern--because of the manner in which the textblock is bound to the paper cover, most mass market paperbacks can only be opened a tiny bit without breaking the spine (as the image below, from Powell's Books, demonstrates):
Against these caveats must be set the very real advantages of paperbacks: they are easily portable; they often carry particularly attractive cover art; they do not take up a lot of shelf space; they are inexpensive.
To better understand the pros and cons of collecting PBOs, it is perhaps best that we explore a bit the history of paperbacks....