I've posted this elsewhere, but here's a list of public domain (free) billiard books.
Anything published 1925 or earlier is now public domain. Here are my favorite historic billiards books:
1. Phelan, Michael, Billiards without a Master (1850). Phelan's halls made billiards popular across America. This book was reprinted and revised for the next thirty years:
2. Collender, Hugh. Modern Billiards (1880). Collender owned the #2 billiard table company in America and his book was very popular in the last few decades of the 19th century. Brunswick eventually bought Collender's company (forming Brunswick, Balke, and Collender) and continued to publish this book until 1909.
3. Daly, Maurice. Daly's Billiard Book (1913). Daly was one of the top players and the top teacher of the early 20th century. Hoppe was one of his students. His book was one of the last great texts of the carom era.
4. Vignaux, Maurice. Le Billiard (1889). Vignaux "The Lion of France" was the top carom player in the days before Hoppe. He helped make carom popular in the U.S. when he won the 1874 U.S. national championship, the first year it moved away from four ball.