When you buy a hard-cover book in a bookstore, the width of the spine matches exactly the number of pages. Not so with Blurb. Only a limited number of spine widths are offered, so the result can be a spine that is considerably wider than necessary.
The result can be quite ugly. See my previous post in the old forums http://forums.blurb.com/forums/5/topics/13061
I've done some detective work and discovered that for the books I make (standard landscape, premium lustre paper) there are in fact five widths, as follows:
Thinnest : 20-50 pages Thin : 52-100 pages Medium: 102-160 pages Thick: 162-200 pages Very thick: 202-240 pages
As a result, if you create a book with, say 100 pages, it will fit snugly in its spine, but if it's 102 the spine will be rather too large, intended for up to 160.
I provide this information to assist potential book makers. Others may care to provide similar information for books with other paper types. (But I won't hold my breath, as these forums are Very Quiet Places.)
Perhaps someone at Blurb could provide further details?