The US Power Squadron back in the pre-GPS 1980s used 'The American Practical Navigator-An Epitome of Navigation (1966)' as their primary reference book. My grandfather ran one of the courses. Amazon(US) has a reprint of the 2002 version:

http://www.amazon.com/The-American-Practical-Navigator-Bowdi...

But you can study it online gratis at:

https://en.wikisource.org/wiki/The_American_Practical_Naviga...