In this thread, I assume we are talking about books primarily intended
to aid in identifying shells, and secondarily to give further detailed
information about species.
 
While numerous illustrations of a species, including different views and
sculpture details is very helpful in the final determination of a
species, it often makes a initial search for a group of shells that
might be the shell that is to be identified difficult.  Abbott & Dance's
"Compendium of Seashells" is very useful for a beginner, or a person
with a poor memory to find the family and genus for detail study.
 
For this reason, it is very helpful for a shell book to have one
smaller photo of each species in a few pages to serve as a visual index.
 Then pile on as much information as desirable in the detailed pages.
In this "index" it is also helpful to have similar shells grouped,
repeating a species if it falls into two groups.
 
As to having illustrations to scale, I disagree.  I would rather have an
illustration that you can identify from than a small dot that is of no
value when a Charonia tritonis is on the same page as a triphorid.  All
photos of the same good size with an indication of length and name of
species should satisfy the desire to identify by size.
 
Aloha,
 
Wes