A basic description of a bookbinding usually contains information about where and when a book was bound, what material in which colour was used to cover it, and how it was decorated. This information is generally presented in the form of: 'In a time, place, tooled/otherwise decorated, colour, material ... binding'. Examples of full written descriptions can be found in The Henry Davis Gift, by M.M. Foot (London, 1978-1983).
The Preservation Advisory Centre produces useful leaflets notably Understanding and caring for bookbindings.
At the British Library, you are not normally permitted to make pencil rubbings of bookbindings. If you have a special reason for doing so, however, please apply in advance to the Curator of Bookbindings.