Bob Katz wrote several editions on mastering, however every edition has become somewhat dated. You can still learn the basics...just avoid the dated stuff.

I think a good way to learn is to participate or just read the feedback in the WUMP threads, after downloading the files. I went to look, but it seems users here have lost interest? Regardless, you can still download, (always .wav files) listen, and learn from the written feedback. If you spend much time in these, you get a sense of who to really take to heart. The noise level can ruin these pretty fast at times, but you get an idea of who to ignore. Anyway, with Brads forum long ago, it was always refreshing and educational to read others opinions.

Here is the gearspace edition

This is a little background:
'WUMP' Proposal: A friendly mastering shootout

Here is WUMP #2 :
WUMP #2

WUMP #3
WUMP #3

I guess there wasn't much interest after #3 ?

Theres a wealth of WUMP posts in Brad Blackwoods old forum as well.