An imprimatur is the official seal of approval from the Church that a book does not contain anything contrary to the faith. Here are the official requirements:
Can. 827 §1. To be published, catechisms and other writings pertaining to catechetical instruction or their translations require the approval of the local ordinary, without prejudice to the prescript of can. 775, §2.
§2. Books which regard questions pertaining to sacred scripture, theology, canon law, ecclesiastical history, and religious or moral disciplines cannot be used as texts on which instruction is based in elementary, middle, or higher schools unless they have been published with the approval of competent ecclesiastical authority or have been approved by it subsequently.
§3. It is recommended that books dealing with the matters mentioned in §2, although not used as texts in instruction, as well as writings which especially concern religion or good morals are submitted to the judgment of the local ordinary.
Whenever a book has an imprimatur we will put that information on the item detail page on our site. This is just another way we help you make informed decisions about your purchases. To see if a book has an imprimatur look for the red Cross of Malta on the item's page. When you put your mouse over the cross a small window with the imprimatur information will pop up.
We are currently working on getting imprimatur information for our books in the system. Until this large project is complete, some books that do carry imprimaturs will not display them.
Please note that the lack of an imprimatur does not make a book bad, it simply falls into the third category of books listed above. Due to the large quantity of titles published each year and a lack of resources within a diocese to properly review all books, many authors do not seek an imprimatur on books that fall into the third category.
For some more information on imprimaturs, we recommend visiting Jimmy Akin's blog.