Good topic. Over a decade ago, I started to put together a baseball library. I searched for "baseball library" and at the time found the following two links.
http://www.sandlotshrink.com/bookoldbb.htm
http://www.kypris.com/Baseball/bb-essential.html
I acquired and read quite a few of the titles. Some of the titles mentioned on the thread made the list.
As mentioned more than once The Glory of Their Times by Ritter is one of the best (if not the best) baseball books ever written. After reading the book, pick up the cd's of Ritter's actual interviews.
Other good reads:
Dollar Sign on the Muscle
The Fireside books
Men at Work
Veeck as in Wreck
Prophet of the Sandlots
Baseball as I have known it
Some are called Clowns
Baseball when the grass was real
The Politics of Glory