These are three different terms.

A key cel refers to any cel based off of a douga 
that was based off of a genga.  The sequence 
number on the douga is usually circled.

An end cel is the last cel in a sequence.  It is 
also a key cel.  Sometimes it is marked with 
an 'E' or an 'End'.

A book cel is a cel with a multi layer background 
or foreground (or combo) elements.  I've seen 
this done with two sheets of background paper, 
with one sheet of background paper and a cut out 
of background paper, and with a background and 
a 'foreground' cel element that was painted akin 
to a background.  There are probably other 
varieties as well.

This was a bit brief.  I hope it helps.  I 
encourage you to search the archives of this 
forum and various cel FAQs for further 
information. :)

Keys