I can't personally recommend a book because I haven't read any books on
Excel 2007. The best way to select a book is to go to a book store and thumb
through them to see if they meet your needs. With that said, here's a good
listing of books:

http://contextures.com/xlbooks.html

Besides the Step-by-Step recommended by T. Valko, I'd also look at the 8-in-1
edition of
Excel 2007 Desk Reference for Dummies by Greg Harvey,
http://www.amazon.com/Excel-2007-Reference-Dummies-Computer/dp/0470037385/ref=pd
_lpo_k2_dp_k2a_3_img/102-6750296-4164126

I recommended the 2003 version of that one to several clients because it
takes them from ground zero into several other areas (hence the 8-in-1 part
of it).  A better value than his similar book with just the introduction to
Excel in it (which the 8-in-1 includes).

Another that looks like it should be good, based on 'past performance' by
the author, would be John Walkenbach's "Quick Reference" for Excel 2007 (for
Dummies)
http://www.amazon.com/Excel-Dummies-Quick-Reference-Computer/dp/0470046716/ref=p
d_lpo_k2_dp_k2a_2_txt/102-6750296-4164126