It isn't strictly liturgical of course, having been used in all manner of books at one time, but it is one of the arts we tend to forget about in terms of our consideration of beautiful items within the liturgy. That art is that of book binding and decoration. 
The middle ages was one of great colour generally. It was seen not only in its liturgical books, but also on the walls of its cathedrals and in its statuary, which were painted and covered in various designs. (This was a subject I spoke about in the summer on the role of colour in the liturgical arts.)
Various eras, including quite recently, also took great care to bind their books with beautiful gilt designs as well which spoke of the permanency and importance of the content of those books (even where that may not have been the case). It was an art that A.W.N. Pugin didn't neglect in his pursuit of a revival of the spirit of the mediaeval arts.
Eventually the binding of books became more plain as a general rule. Likewise, even beautiful engravings in books became less and less common. This tendency became accelerated in the past few decades, and today, many of our liturgical books, either inside or out, are less impressive and inspiring in design than even a common Encyclopedia Brittanica from the turn of the 20th century.
Of course, this isn't strictly unique to books. In many regards our liturgical art, be it statuary, architecture, art and design has become far more plain and simple in design.
Where is all of this leading? Well, quite simply to a remembrance of the beauty that can be with regard to books, and ought to be present with regard to our liturgical books.
In that vein, I thought readers might be interested in this interactive tutorial (sent by a reader) which demonstrates the process which was involved in the creation and decoration of mediaeval illuminated manuscripts: Making Art: Medieval Manuscripts
