I second ralph's recommendations highly, they are both excellent books for taking your CSS to the next level.
Also, I really like Chrome's Inspector, it's a life saver.
I think the easiest way is to just play around with them.
The most important properties which you should really master are:
background, font, width, height, float, clear, position, border, padding, margin, color
Those elements will likely be used many many times throughout your site. Others will be used, but not nearly as frequently.
Also, CSS can be a bit finicky and has a lot of "special case" which can throw you at first. That's why it's important to just study other people's CSS as well as play with your own. For example, one that still occassionally throws me for a loop is Collapsing Margins: http://reference.sitepoint.com/css/collapsingmargins
Some things play better with others, so it's eventually just becomes a matter of figuring out what you want to do, then figuring out the best way to accomplish that (which comes from experience).