I'm sure that this topic has been brought up before but as a relative newcomer to the blog and with time to read during the Summer as there are no live games to watch on television I wanted to throw it out there for discussion.
I thought all three John U Bacon books were very good. Starting with Bo Schembechler, moving on to Rich Rodriguez, and ending with the hiring of Jim Harbaugh they are all very informative and well written. I also enjoyed Curt Stephenson's book ("Those Who Stay"). It gives a first hand account of what it is like to be a walkon playing for Bo Schembechler and how hard work and determination pays off. When you can walkon and then catch a touchdown pass in the Rose Bowl it is certainly a very special story.
I forgot to mention. "The University of Michigan Football Scrapbook" by Cohen, Deutsch, and Neft is a really fun book. It has copies of original newspaper clippings of games, photographs of players, and stats. It starts in 1879 and goes up until the end of the 1977 season. The newspaper clippings are great.