(The New York Times is always a great source of news and (usually) thoughtful commentary on literary matters. I thought this brief overview of the differences between the two main types of paperback books out there — mass market and trade — might be of interest to readers and authors alike. So with the indulgence of our friends at the Times, here it is — and by the way, be sure to check out the Times’ various best-seller lists at the end of this post).

Besides being somewhat larger in size, trade paperbacks are generally printed on more expensive paper and with sturdier binding. Because they are more expensive to produce they are higher in price and often (not always) printed in smaller numbers. Unlike mass-market paperbacks, which are usually sold on racks, trade paperbacks are sold in bookstores (“to the trade”) and are shelved with their spines facing out, like hardcovers. Sometimes they are sold on display tables, lying flat so that customers can respond to their cover art. Trade paperbacks may be originals, which are not preceded by a hardcover edition, or reprints of hardcovers. A trade paperback, in short, is the book you’d want to be reading if you were sitting at Les Deux Magots and Simone de Beauvoir was looking straight at you.

In recent years, the distinction between mass-market and trade paperbacks has been eroding. So while content and genre no longer determine whether a book is a mass-market or trade paperback, the book’s size, the quality of its paper, the way it is displayed, its price, the way it is distributed and the place it is sold all go into the definition. R. R. Bowker, the company that assigns International Standard Book Numbers (ISBNs) to books published in the United States, has developed codes and identifiers that define which books are trade paperbacks and which are mass-market.

You may still wonder why we decided to separate the mass-market and trade best-seller lists. The reason is that mass-market books — no surprise — tend to sell in larger numbers than trade. A list based on the number of copies a paperback sells will usually be dominated by mass-market. (Similarly, advice and self-help books sell more than most general nonfiction, and they dominated the nonfiction best-seller list until they got their own property in 1984.) But the Book Review — like most review media — focuses on trade fiction. These are the novels that reading groups choose and college professors teach. On the paperback best-seller list for Sept. 16, the week before we switched to the new system, only 7 of the 15 entries were trade fiction, but the new list of Sept. 23 presents 20 trade paperbacks. The seven books that made the list the week before, including “The Kite Runner” and “The Alchemist,” are still there, near the top of the list. But now there’s also room for Irène Némirovsky’s “Suite Française” and Kiran Desai’s “Inheritance of Loss.” And there’s a fuller listing for mass-market novels and paperback nonfiction as well.