If you're serious, the Bob Katz book is excellent.
Also check out his honour roll of recordings on his Website. Although he tends towards the conservative end of mastering, you need to build up a collection of some excellent recordings with which to compare your work. All things Bob Katz are at
http://www.digido.comIf you need to train your ears to recognize and focus like a lens on various areas of the spectrum, a good place to start would be a set of CD's called Golden Ears, from Dave Moulton of Moulton Labs.
http://www.moultonlabs.com/full/product01/If you are a musician with good ears - I'm one of those perfect pitch assholes
check out a chart that shows the frequencies of the musical pitches you are used to hearing. You need to start thinking "oh that's 500 HZ" not "oh that's around 4th octave B natural".
http://www.phy.mtu.edu/~suits/notefreqs.htmlAnd for god sake, before you worry about expensive monitors, get the acoustics in your room sorted out. The amateurs all overlook that step, and I'd say that room response accounts for about 50% of what you are hearing.
Speaking of monitors, you'll be listening to unmastered recordings, with intact transients. You want something that can really let you hear those very short peaks. A sealed box type speaker will do that more likely than ported designs which tend to ring.
Re. room acoustics, read:
www.ethanwiner.com/acoustics.htmland check out
www.realtraps.comor you can build your own traps and pannels and save lots of $$$.
Although I can't find the link, Brad Blackwood runs a mastering forum you may want to join.
and, Berkleemusic Online runs a 12-week mastering course that uses the Bob Katz book as textbook.