There are a few that pop to mind..
Remember one name: David Vizard
How to Modify Your Mini - David Vizard $54.95
http://www.pitstop.net.au/pitstop/page/marques_mini/8313.html
Mini Workshop Manual aka 'Black Blook' - Leyland Aust. $69.00
http://www.pitstop.net.au/pitstop/prog/OpenDocument.cgi?id=V0X7S13PUM&PLU=1076
Tuning The A - Series Engine (3rd Edition) - David Vizard $59.95
http://www.pitstop.net.au/pitstop/prog/OpenDocument.cgi?id=V0X7S13PUM&PLU=8858
Haynes Workshop Manual - Mini 1969-2001 $55.95
http://www.pitstop.net.au/pitstop/prog/OpenDocument.cgi?id=V0X7S13PUM&PLU=2839
Also Good Reading:
A Collection Of Articles from Wheels, Motor etc at the time...
Mini Gold Portfolio:
http://www.pitstop.net.au/pitstop/prog/OpenDocument.cgi?id=V0X7S13PUM&PLU=7207
For fun...
Ultimate Mini Extreme DVD $49.95
http://www.pitstop.net.au/pitstop/prog/OpenDocument.cgi?id=V0X7S13PUM&PLU=15700
Anything by Vizard is highly recommended..
To start off i'd get the Haynes Manual and How to Modify Your Mini (It's covers more, but in less detail..but still very good)..Leyland Black Book wouldn't hurt either once you start getting into things..