Felix said:
So there's no reason to call the book anything but popular. It follows the adage: When In Doubt, Declare Success!
Well, not exactly. The evidence we have to date is that the Bo9S is one of the better sellers at Amazon. It seems that WotC is declaring success due to success.
People should check out Amazon's Category Rankings: Entertainment / Puzzles & Games / Role Playing & Fantasy / Dungeons & Dragons
What Sales Rank Means
As an added service for customers, authors, publishers, artists, labels, and studios, we show how items in our catalog are selling. The lower the number, the higher the sales for that particular item. The calculation is based on Amazon.com sales and is updated each hour to reflect recent and historical sales of every item sold on Amazon.com. We hope you find the Amazon.com Sales Rank interesting!
What Category Sales Ranks Mean
While Amazon.com Sales Rank is a good indicator of how well a product is selling overall, it doesn't always indicate how popular an item is among other similar items. Our category sales ranks were created to highlight those categories where an item really stands out.
Bo9S is ranked 20 on that list.
#2 Rules Compendium Oct 2007
#4 Players Handbook Jul 2003
#5 Core Rulebook Set Sep 2003
#8 Magic Item Comp Mar 2007
#9 Dungeon Master's Guide Jul 2003
#10 Monster Manual Jul 2003
#11 Complete Scoundrel Jan 2007
#12 Player's Handbook II May 2006
#13 Complete Adventurer Jan 2005
#14 Complete Warrior Dec 2003
#15 Complete Arcane Nov 2004
#16 Monster Manual IV Jul 2006
#17 Dungeon Master's Guide II Jun 2005
#20 Book of Nine Swords Aug 2006
20 out of 790 results (many of which are paperback novels)
Realizing that recently released books will have better rankings than older books, it is still amazing that Bo9S beat out the entire Eberron hardbounds, the entire Forgotten Realms hardbounds, a few of the Complete Series (Complete Divine, Complete Psionics), the Races books, Unearthed Arcana, etc. WotC has put out more than a hundred hardbound books and Bo9S has beat most of them.
The Sales Rankings are misleading compared to the Category Rankings (which are still skewed somewhat by recent releases). In 6 months, Bo9S might be higher than Rules Compendium, Magic Item Compendium, and Complete Scoundrel due to how Amazon ranks these. Considering that it has been on the list for 1 1/2 years, it's pretty high up.
And statistically, Amazon is big enough that the rankings here are probably very close to WotC's rankings (except for the recent sales aspect).
All in all, the good sales of Bo9S does not appear to be a myth after all.