Taste of Cinema has posted a nice list of the 10 best "Making Of" documentaries, and they are really good choices, so I'm crossposting it here (with due attribution to Taste of Cinema, source link below).
1. Hearts of Darkness: A Filmmaker's Apocalypse (1991)
2. Burden of Dreams (1982)
3. Man with a Movie Camera (1929)
4. Dangerous Days: Making Blade Runner (2007)
5. Making the Shining (1980)
6. Best Worst Movie (2009)
7. Full Tilt Boogie (1997)
8. American Movie (1999)
9. Overnight (2003)
10. Lost In La Mancha (2002)
I'm a big fan of behind the scenes documentaries. Seeing how a great (or not so great) movie was made is sometimes even more interesting than the actual movie. If you want more information and behind the scenes pictures, follow the source link: http://www.tasteofcinema.com/2014/10-best-making-of-documentaries-you-need-to-watch