Pedigree Books and Reference Material

Greathouse Publications - Offers a great book called "Thoroughbred Pedigree Reference Sourcebook" which traces the sire lines of over 40,000 horses back to the foundation sires. Very useful book!
Breeding & Pedigree Books - Pedigree and Breeding books from Racingbooks.com
Richard Ulbrich's Peerage of Racehorses - A book which graphically charts out the most famous horses and thoroughbred lines.
S&M Enterprises - Books, pedigrees, a newsletter, and even stand some Arabian stallions at stud.
Susan Larkin's Thoroughbred Books - A great collection of books & research materials for pedigree researchers.
TBHeritage Research Resources - Reviews, suggested reading material and more from Thoroughbred Heritage.
Thoroughbred Legends - From the bloodhorse. A series chronicalling some of the great thoroughbreds of the last 100 years.
The X-Factor - A book explaining the genetic link that gives some horses larger hearts than others.