The spine, as part of the cover, is usually decorated in a similar style as the cover. The spine may be tooled and decorated like Al-Bukhari and Juris, or plain like Biblia Latina or Sermones Angelicii.
The most common unifying feature is the presence of the title and/or author of the volume contained between the covers. This information may be written by hand in ink, as with Sermones or Biblia Latina, or stamped into or written on a label that is then affixed to the leather of the spine.
While title or author were frequently put on the spine of the cover in some fashion, this was not always the case. As you can see from the image of Livy, it was also possible for the spine to be completely plain with only the bands from the binding for "decoration".
The spine of books bound in parchment wrappers is generally left plain or, as in the case Sermones Angelicii, labelled by hand in ink with the author and/or title.