A book-contract clause that indicates that you must give this publisher an option to buy your next book before you submit it elsewhere. When your next book is ready to submit for publication, it must go first to your original publisher. The clause should also indicate how long they have to decide whether or not they want to publish the next book. That time frame is usually somewhere between 30 and 90 days (try to keep it at no more than 30 days). Where an option clause can become a problem is when it indicates that if the original publisher turns it down—but you then find another publisher interested in buying it—you must then take it back to the original publisher and give them another chance to accept it before you can accept the second offer. Another problem with this clause is that the author must offer the book to the publisher—but the publisher is not obligated to buy it. For many years this clause was omitted from most book contracts because it was not considered fair to the author. However, in the last few years it has been working its way back into many book contracts. Although the author can ask for it to be removed—and many do—many publishers will refuse to do so. If the publisher refuses to remove it, be sure that it indicates that the next book (if accepted) will be published on terms to be negotiated—not on the same terms as the current one. You also want this clause to indicate that the publisher will make a decision based on a book proposal—not on a completed manuscript. Also, you want to be free to accept an offer from another publisher—even if these terms are worse than what the original publisher offers. In most cases you would not accept the lesser offer, unless you have a reason—such as believing the new publisher will do a better job of promotion.
From the publisher's point of view, they believe that you owe them a certain amount of loyalty because they made the investment in your previous book—and if the first book does well in the marketplace, there may not be any reason to switch publishers. Some authors stay with the same publisher for their entire writing career. For a perspective on dealing with multiple option clauses, go to: http://literaticat.blogspot.com/2010/09/options-options-options.html.