There is a great thread over on the the forums at the Man Without Ties website titled "The PW / Mats Library Shelf" and I'm gonna plagirize it.
Here are my top 5 selections:
1) The Replacements - All Over But The Shouting - an Oral History by Jim Walsh
Great snippets of Mats lore
2) The Second Greatest Story Ever Told by Gorman Bechard
Fictional account of the daughter of God - and she's a Replacements fan - go figure
3) Our Band Could Be Your Life - Scenes from the American Indie Underground 1981 - 1991 by Michael Azzerad
The Mats influence Nirvana? - who wouldda guessed
4) 31 Songs by Nick Hornby
An homage to Westerberg's "Born for Me" encompasses chapter 12
5) Let it Be by Colin Meloy (in the 33 1/3 series)
Great book, not a huge fan of the author's band