How lucky we are to have Robin Furth's research to guide us through AllWorld. Both volumes of the Concordance are easy-to-use references to things that may be hard for some (like me!) to keep straight over the course of seven volumes of Dark Tower, much less the many books that relate to the Tower.
While I understand that casual readers of the Dark Tower series may not require all of the information included in the Concordance, I still feel these books to be an important addition to any reader's library. If you're like me, and re-read your favorite books time and time again, the opportunity to remind one's self of certain facts significantly enhances the read. Furth makes it easy to do just that -- flip a few pages, get what you need, and go back to the story.
My only fault -- and I will not count it against the publisher -- is that it isn't available in hard cover. I have the entire series, and am now gathering all titles that relate to it... all in hard cover, even some firsts. (Calvin would be proud, but upset that I'd actually READ the Firsts.) I would definitely purchase hard covers of both volumes if they were available.