Hello Jennifer,
Any book wide enough in scope to be arranged by geographic zones
would necessarily not be very comprehensive, and such "overview"
types of books are most useful when arranged by taxonomic
relationships (by families).  The Compendium of Seashells is perhaps
the best known example of such a book.  This book is worldwide in
scope, and therefore necessarily covers only a fraction of the
species in any one family of shells, or in any one geographic area.
Therefore it would be very cumbersome to identify, for example, a
cone shell of unknown origin if you had to check the Panamic section,
the Australian section, the Mediterranean section, the Caribbean
section, etc.  No single work could even attempt to cover all the
species in all the geographic zones.  Moving beyond this general type
of book, you find two principle groups of comprehensive identification
guides - those which deal with one family of shells worldwide, and
those which deal with all families in one well defined geographic
area.  These books are usually the most useful ones for advanced
collectors, because there is a high degree of probability that the
shell you are trying to identify is actually in the book.  In
contrast, trying to identify an unknown Terebra or Cymatium (or
worse, a bivalve) from the Compendium, you start out knowing there is
only a 25% chance (or maybe a 5% chance in the case of a venus clam or
a cockle) that the species you have is included in the book.
THE book for the Panamic Province is Seashells of Tropical West
America by Myra Keen.  It is not the easiest book for a beginning
collector to use.  It is a large book (3 inches thick), with
thousands of small, black and white pictures (mostly photographs but
some line drawings).  Still, there is simply nothing comparable if
you are trying to identify an unknown shell picked up between Baja
and Peru.  This book is available from Amazon.com for $79.50.
Regards,
Paul Monfils