Andrew,
 
As a reader I have just a few requests for any author of a shell family book.
First you really need quality pictures/illustrations/images whatever.  A good
color photo is the best and most desired image but if it is only one inch
square it looses a lot of usefulness.  If there is no color image available
then a black and white or a line drawing is much, much better than nothing.
The compendium is a great reference but the pictures are a bit small and their
isolation makes them somehow more difficult to use -- often a 10 inch shell
looks like it is the same size as a 5 mm shell.  Books that combine shells in a
large picture such as Eisenburg's book are a bit easier to use.  Rombouts book
on pectens is great and the pictures show several species together but the
pictures often show only one side of a bivalve with very different looking
valves or one image of a species which is known to be extremely variable.
Henning and Hemmen's book on Ranellidae is excellent but all of the images are
pen and ink drawings (some of which are very good for identification purposes -
some not so good).  But when the three books are compared, the Ranellidae book
is maybe the best when you are trying to identify an unknown shell because the
authors included very specific, detailed descriptions to go with each species
pictured.  A picture may indeed tell a thousand words but they may not include
the fact that the outer lip has small denticles or the umbilicus is closed or
whatever.  I would also believe a picture with several species has to be
cheaper to print.  Oh, and it helps if the captions are clear for each
picture.  Ponder's book on xenophora has great descriptions and pretty nice
black and white photographs but the captions are hard to read and match with
the correct image.
 
Well, you now have my opinion and we all know it is worth what you paid for
it.  But you did ask.
 
Oh yes, a book on a shell family ought to have some details about the family as
a whole.  One of the early paragraphs should talk about physical shell
structure, animal characteristics, behavior, habitats, history, and anything
else pertaining to the family under discussion.  An image or three of a living
animal would also be really nice in this section.  Of the family books I've
mentioned; Rombouts has nothing on the family as a whole, Henning and Hemmen
have a page and Ponder has 15 pages with anatomy, fossil history, taxonomy, and
biology (feeding, reproduction, habits, shell structure).
 
Now after saying all of this, some of which sounds like criticism (and it is),
I have to say my hat is off to anybody who attempts to write such a book (as
for Art and company, I seriously think they may be crazy).  They will never
make any real money off of such a book, as soon as it is published some joker
will mess with the taxonomy or invalidate an entire genus or something similar,
people like me will criticize it, and there will be editorial "changes" or
mistakes for which they will be blamed but over which they have no control.
But thankfully some people will write these books anyway -- thank goodness, for
next to the shells I think I like the books.  Without these books we would all
have meaningless collections and would almost be forced to specialize in very
small areas of the shell world.  So I will be in line for the epitonium book
when it comes out or the new xenophora book when it comes out.  Now if we can
just talk someone into taking on the neritidae, or the muricidae, or the
turridae (only a mad man) we will be getting some where.
 
Another unbiased opinion from the man on the street
 
Tom Eichhorst in New Mexico, USA (I promise, nothing more on the list for a
couple of days, I'll give everyone a break)