In the bookselling business, a strip is a book that's literally not worth the paper it's printed on.
In general, books come in three different formats: hardcovers, trade paperbacks and mass-market paperbacks. Hardcovers and trades are expensive to produce and are therefore sent back to the store's distributors for a merchandise credit if they're not sold. Mass markets are a different animal - they're printed on extremely cheap paper, are poorly bound and generally don't last more than six months if stored in direct sunlight. They're designed to be read and forgotten; most romance, horror, science fiction serials and budget classics are printed as mass markets. They're called mass markets because they account for a huge percentage of book sales and represent (you guessed it) the mass market.
Mass markets are so cheap to produce, in fact, and have such a short lifespan that they're not worth the cost of shipping to the book distributors if they languish on the shelves. They are therefore designated as strips - when it comes time to return them, the covers are stripped from the books and sent back to the wholesalers without their pages in exchange for a credit. The books themselves are (or at least should be) destroyed.
We don't actually destroy our strips at my bookstore - we wrap them in wrapping paper and use them for displays, we use them as doorstops or we use them as weapons. We do NOT, however, sell them.
Buying books without their covers is actually illegal - it qualifies as a copyright violation. That's why most mass markets have some variation of the following text printed on their copyright pages:
If you purchased this book without a cover you should be aware that this book is stolen property. It was reported as "unsold and destroyed" to the publisher and neither the author nor the publisher has received any payment for this "stripped book."